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• SQL SERVER – Three Puzzling Questions – Need Your Answer

  - by pinaldave

  Last week I had asked three questions on my blog. I got very good response to the questions. I am planning to write summary post for each of three questions next week. Before I write summary post and give credit to all the valid answers. I was wondering if I can bring to notice of all of you this week. Why SELECT * throws an error but SELECT COUNT(*) does not This is indeed very interesting question as not quite many realize that this kind of behavior SQL Server demonstrates out of the box. Once you run both the code and read the explanation it totally makes sense why SQL Server is behaving how it is behaving. Also there is connect item is associated with it. Also read the very first comment by Rob Farley it also shares very interesting detail. Statistics are not Updated but are Created Once This puzzle has multiple right answer. I am glad to see many of the correct answer as a comment to this blog post. Statistics are very important and it really helps SQL Server Engine to come up with optimal execution plan. DBA quite often ignore statistics thinking it does not need to be updated, as they are automatically maintained if proper database setting is configured (auto update and auto create). Well, in this question, we have scenario even though auto create and auto update statistics are ON, statistics is not updated. There are multiple solutions but what will be your solution in this case? When to use Function and When to use Stored Procedure This question is rather open ended question – there is no right or wrong answer. Everybody developer has always used functions and stored procedures. Here is the chance to justify when to use Stored Procedure and when to use Functions. I want to acknowledge that they can be used interchangeably but there are few reasons when one should not do that. There are few reasons when one is better than other. Let us discuss this here. Your opinion matters. Reference: Pinal Dave (http://blog.SQLAuthority.com) Filed under: Pinal Dave, PostADay, Readers Contribution, Readers Question, SQL, SQL Authority, SQL Performance, SQL Puzzle, SQL Query, SQL Scripts, SQL Server, SQL Tips and Tricks, SQLAuthority News, SQLServer, T SQL, Technology

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• SQLAuthority News – New Book Released – SQL Server Interview Questions And Answers

  - by pinaldave

  Two days ago, on birthday of my blog – I asked simple question – Guess! What is in this box? I have received lots of interesting comments on the blog about what is in it. Many of you got it absolutely incorrect and many got it close to the right answer but no one got it 100% correct. Well, no issue at all, I am going to give away the price to whoever has the closest answer first in personal email. Here is the answer to the question about what is in the box? Here it is – the box has my new book. In fact, I should say our new book as I co-authored this book with my very good friend Vinod Kumar. We had real blast writing this book together and had lots of interesting conversation when we were writing this book. This book has one simple goal – “master the basics.” This book is not only for people who are preparing for interview. This book is for every one who wants to revisit the basics and wants to prepare themselves to the technology. One always needs to have practical knowledge to do their duty efficiently. This book talks about more than basics. There are multiple ways to present learning – either we can create simple book or make it interesting. We have decided the learning should be interactive and have opted for Interview Questions and Answer format. Here is quick interview which we have done together. Details of the books are here The core concept of this book will continue to evolve over time. I am sure many of you will come along with us on this journey and submit your suggestions to us to make this book a key reference for anybody who wants to start with SQL server. Today we want to acknowledge the fact that you will help us keep this book alive forever with the latest updates. We want to thank everyone who participates in this journey with us. You can get the books from [Amazon] | [Flipkart]. Read Vinod‘s blog post. Do not forget to wish him happy birthday as today is his birthday and also book release day – two reason to wish him congratulations. Reference: Pinal Dave (http://blog.SQLAuthority.com) Filed under: Best Practices, Data Warehousing, Database, Pinal Dave, PostADay, SQL, SQL Authority, SQL Interview Questions and Answers, SQL Query, SQL Server, SQL Tips and Tricks, SQLAuthority Book Review, SQLAuthority News, SQLServer, T SQL, Technology

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• SQL SERVER – Move Database Files MDF and LDF to Another Location

  - by pinaldave

  When a novice DBA or Developer create a database they use SQL Server Management Studio to create new database. Additionally, the T-SQL script to create a database is very easy as well. You can just write CREATE DATABASE DatabaseName and it will create new database for you. The point to remember here is that it will create the database at the default location specified for SQL Server Instance (this default instance can be changed and we will see that in future blog posts). Now, once the database goes in production it will start to grow. It is not common to keep the Database on the same location where OS is installed. Usually Database files are on SAN, Separate Disk Array or on SSDs. This is done usually for performance reason and manageability perspective. Now the challenges comes up when database which was installed at not preferred default location and needs to move to a different location. Here is the quick tutorial how you can do it. Let us assume we have two folders loc1 and loc2. We want to move database files from loc1 to loc2. USE MASTER; GO -- Take database in single user mode -- if you are facing errors -- This may terminate your active transactions for database ALTER DATABASE TestDB SET SINGLE_USER WITH ROLLBACK IMMEDIATE; GO -- Detach DB EXEC MASTER.dbo.sp_detach_db @dbname = N'TestDB' GO Now move the files from loc1 to loc2. You can now reattach the files with new locations. -- Move MDF File from Loc1 to Loc 2 -- Re-Attached DB CREATE DATABASE [TestDB] ON ( FILENAME = N'F:\loc2\TestDB.mdf' ), ( FILENAME = N'F:\loc2\TestDB_log.ldf' ) FOR ATTACH GO Well, we are done. There is little warning here for you: If you do ROLLBACK IMMEDIATE you may terminate your active transactions so do not use it randomly. Do it if you are confident that they are not needed or due to any reason there is a connection to the database which you are not able to kill manually after review. Reference : Pinal Dave (http://blog.SQLAuthority.com) Filed under: PostADay, SQL, SQL Authority, SQL Backup and Restore, SQL Query, SQL Server, SQL Tips and Tricks, T SQL, Technology

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• SQL SERVER – Detecting guest User Permissions – guest User Access Status

  - by pinaldave

  Earlier I wrote the blog post SQL SERVER – Disable Guest Account – Serious Security Issue, and I got many comments asking questions related to the guest user. Here are the comments of Manoj: 1) How do we know if the uest user is enabled or disabled? 2) What is the default for guest user in SQL Server? Default settings for guest user When SQL Server is installed by default, the guest user is disabled for security reasons. If the guest user is not properly configured, it can create a major security issue. You can read more about this here. Identify guest user status There are multiple ways to identify guest user status: Using SQL Server Management Studio (SSMS) You can expand the database node >> Security >> Users. If you see the RED arrow pointing downward, it means that the guest user is disabled. Using sys.sysusers Here is a simple script. If you notice column dbaccess as 1, it means that the guest user is enabled and has access to the database. SELECT name, hasdbaccess FROM sys.sysusers WHERE name = 'guest' Using sys.database_principals and sys.server_permissions This script is valid in SQL Server 2005 and a later version. This is my default method recently. SELECT name, permission_name, state_desc FROM sys.database_principals dp INNER JOIN sys.server_permissions sp ON dp.principal_id = sp.grantee_principal_id WHERE name = 'guest' AND permission_name = 'CONNECT' Using sp_helprotect Just run the following stored procedure which will give you all the permissions associated with the user. sp_helprotect @username = 'guest' Disable Guest Account REVOKE CONNECT FROM guest Additionally, the guest account cannot be disabled in master and tempdb; it is always enabled. There is a special need for this. Let me ask a question back at you: In which scenario do you think this will be useful to keep the guest, and what will the additional configuration go along with the scenario? Note: Special mention to Imran Mohammed for being always there when users need help. Reference: Pinal Dave (http://blog.sqlauthority.com) Filed under: PostADay, SQL, SQL Authority, SQL Query, SQL Security, SQL Server, SQL Tips and Tricks, T SQL, Technology

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• SQLAuthority News – I am Presenting 2 Sessions at TechEd India

  - by pinaldave

  TechED is the event which I am always excited about. It is one of the largest technology in India. Microsoft Tech Ed India 2011 is the premier technical education and networking event for tech professionals interested in learning, connecting and exploring a broad set of current and soon-to-be released Microsoft technologies, tools, platforms and services. I am going to speak at the TechED on two very interesting and advanced subjects. Venue: The LaLiT Ashok Kumara Krupa High Grounds Bangalore – 560001, Karnataka, India Sessions Date: March 25, 2011 Understanding SQL Server Behavioral Pattern – SQL Server Extended Events Date and Time: March 25, 2011 12:00 PM to 01:00 PM History repeats itself! SQL Server 2008 has introduced a very powerful, yet very minimal reoccurring feature called Extended Events. This advanced session will teach experienced administrators’ capabilities that were not possible before. From T-SQL error to CPU bottleneck, error login to deadlocks –Extended Event can detect it for you. Understanding the pattern of events can prevent future mistakes. SQL Server Waits and Queues – Your Gateway to Perf. Troubleshooting Date and Time: March 25, 2011 04:15 PM to 05:15 PM Just like a horoscope, SQL Server Waits and Queues can reveal your past, explain your present and predict your future. SQL Server Performance Tuning uses the Waits and Queues as a proven method to identify the best opportunities to improve performance. A glance at Wait Types can tell where there is a bottleneck. Learn how to identify bottlenecks and potential resolutions in this fast paced, advanced performance tuning session. My session will be on the third day of the event and I am very sure that everybody will be in groove to learn new interesting subjects. I will have few give-away during and at the end of the session. I will not tell you what I will have but it will be for sure something you will love to have. Please make a point and reserve above time slots to attend my session. Reference: Pinal Dave (http://blog.SQLAuthority.com) Filed under: About Me, Pinal Dave, PostADay, SQL, SQL Authority, SQL Query, SQL Server, SQL Tips and Tricks, SQL Wait Stats, SQL Wait Types, T SQL, Technology Tagged: SQL Extended Events

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• SQL SERVER – Solution – 2 T-SQL Puzzles – Display Star and Shortest Code to Display 1

  - by pinaldave

  Earlier on this blog we had asked two puzzles. The response from all of you is nothing but Amazing. I have received 350+ responses. Many are valid and many were indeed something I had not thought about it. I strongly suggest you read all the puzzles and their answers here - trust me if you start reading the comments you will not stop till you read every single comment. Seriously trust me on it. Personally I have learned a lot from it. Let us recap the puzzles here quickly. Puzzle 1: Why following code when executed in SSMS displays result as a * (Star)? SELECT CAST(634 AS VARCHAR(2)) Puzzle 2: Write the shortest code that produces results as 1 without using any numbers in the select statement. Bonus Q: How many different Operating System (OS) NuoDB support? As I mentioned earlier the participation was nothing but Amazing. I will write about the winners and the best answers in short time. Meanwhile I will give to the point answers to above puzzles. Solution 1: When you convert character or binary expressions (char, nchar, nvarchar, varchar,binary, or varbinary) to an expression of a different data type, data can be truncated, only partially displayed, or an error is returned because the result is too short to display. Conversions to char, varchar, nchar, nvarchar, binary, and varbinary are truncated, except for the conversions shown in the following table. Reference of the text and table from MSDN. Solution 2: The shortest code to produce answer 1 : SELECT EXP($) or SELECT COS($) or SELECT DAY($) When SELECT $ it gives us the result as 0.00 and the EXP of the same is 1. I believe it is pretty neat. There were plenty other answers but this was the shortest. Another shorter answer would be PRINT EXP($) but no one has proposed that as in original Question I have explicitly mentioned SELECT in the original question. Bonus Answer: 5 OS: Windows, MacOS, Linux, Solaris, Joyent SmartOS Reference Please do read every single comment here. Do leave a comment which one do you think is the best comment out of all the comments. Meanwhile if there is a better solution and I have missed it do let me know as we still have time to correct it. I will be selecting the winner before the weekend as I am going through each and every of 350 comment. I will be selecting the best comments along with the winning comment. If our selection matches – one of you may still win something cool.  Reference: Pinal Dave (http://blog.SQLAuthority.com) Filed under: PostADay, SQL, SQL Authority, SQL Puzzle, SQL Query, SQL Server, SQL Tips and Tricks, SQLServer, T SQL, Technology Tagged: NuoDB

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• SQLAuthority News – A Quick Note on @Pluralsight Video – Call Me Maybe Developer Way

  - by pinaldave

  I write a lot about how important learning and training is.  Any of my readers will know that I think the key to success is staying current with your education and taking very opportunity to increase your “tool kit” of skills.  I hope that I have not made the impression that it is all in the employees hands to make sure they are happy and satisfied at their jobs. I also firmly believe that a good boss will make good employees.  A boss who is good at communicating,  and leading, who knows how to nip problem in the bud and allocate resources wisely will have a well-oiled machine.  This means happy employees and a great work environment. It is important to have a healthy work environment because you will not succeed without one.  Successful business will always have the type of environment that fosters creativity and has efficient employees.  A healthy environment doesn’t force employees to produce results, but allows them to progress and create the results themselves. The result of a healthy work environment is that employees will enjoy their work and then work harder.  This can bring the company more revenue, and hopefully the employees will see the result of their hard work in bonuses and raises.  However, money is important but it is certainly secondary – the important part is the dedication of the employees to their work and to their company.  This is the true key to success. Any employee who recognizes this description as their working environment should consider themselves fortunate.  They are allowed to grow and do better, and employees being treated fairly can be a rarity in this world.  One company that I believe adheres to this principle is Pluralsight – as evidenced by this fun video. I have blogged about it earlier. (check out my cameo at 0:37). It was great fun to work with the employees at Pluralsight while making this video.  They are a great bunch and clearly have a great work environment – we wouldn’t have had this much fun if not!  I have to tell you a little bit about making this video.  My wife shot it with her mobile phone, which was certainly a different but exciting experience!  It was hard to get the look of the video right, since I was trying to portray a body builder – this was a little outside of my own personal experience.  I have what I like to call a “healthy” body type, so trying to look extremely fit like some of the other “actors” in this video was a challenge – but I do hope that you all think I succeeded.  All in all, it was great fun to participate in this video and I hope to see my friends at Pluralsight again soon. Reference: Pinal Dave (http://blog.SQLAuthority.com) Filed under: PostADay, SQL, SQL Authority, SQL Query, SQL Server, SQL Tips and Tricks, T SQL, Technology, Video

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• SQL SERVER – NTFS File System Performance for SQL Server

  - by pinaldave

  Note: Before practicing any of the suggestion of this article, consult your IT Infrastructural Admin, applying the suggestion without proper testing can only damage your system. Question: “Pinal, we have 80 GB of data including all the database files, we have our data in NTFS file system. We have proper backups are set up. Any suggestion for our NTFS file system performance improvement. Our SQL Server box is running only SQL Server and nothing else. Please advise.” When I receive questions which I have just listed above, it often sends me deep thought. Honestly, I know a lot but there are plenty of things, I believe can be built with community knowledge base. Today I need you to help me to complete this list. I will start the list and you help me complete it. NTFS File System Performance Best Practices for SQL Server Disable Indexing on disk volumes Disable generation of 8.3 names (command: FSUTIL BEHAVIOR SET DISABLE8DOT3 1) Disable last file access time tracking (command: FSUTIL BEHAVIOR SET DISABLELASTACCESS 1) Keep some space empty (let us say 15% for reference) on drive is possible (Only on Filestream Data storage volume) Defragement the volume Add your suggestions here… The one which I often get a pretty big debate is NTFS allocation size. I have seen that on the disk volume which stores filestream data, when increased allocation to 64K from 4K, it reduces the fragmentation. Again, I suggest you attempt this after proper testing on your server. Every system is different and the file stored is different. Here is when I would like to request you to share your experience with related to NTFS allocation size. If you do not agree with any of the above suggestions, leave a comment with reference and I will modify it. Please note that above list prepared assuming the SQL Server application is only running on the computer system. The next question does all these still relevant for SSD – I personally have no experience with SSD with large database so I will refrain from comment. Reference: Pinal Dave (http://blog.sqlauthority.com) Filed under: PostADay, SQL, SQL Authority, SQL Performance, SQL Query, SQL Server, SQL Tips and Tricks, T SQL, Technology

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• SQL SERVER – Transaction Log Full – Transaction Log Larger than Data File – Notes from Fields #001

  - by Pinal Dave

  I am very excited to announce a new series on this blog – Notes from Fields. I have been blogging for almost 7 years on this blog and it has been a wonderful experience. Though, I have extensive experience with SQL and Databases, it is always a good idea that we consult experts for their advice and opinion. Following the same thought process, I have started this new series of Notes from Fields. In this series we will have notes from various experts in the database world. My friends at Linchpin People have graciously decided to support me in my new initiation.  Linchpin People are database coaches and wellness experts for a data driven world. In this very first episode of the Notes from Fields series database expert Tim Radney (partner at Linchpin People) explains a very common issue DBA and Developer faces in their career, when database logs fills up your hard-drive or your database log is larger than your data file. Read the experience of Tim in his own words. As a consultant, I encounter a number of common issues with clients.  One of the more common things I encounter is finding a user database in the FULL recovery model that does not make a regular transaction log backups or ever had a transaction log backup. When I find this, usually the transaction log is several times larger than the data file. Finding this issue is very significant to me in that it allows to me to discuss service level agreements with the client. I get to ask questions such as, are nightly full backups sufficient or do they need point in time recovery.  This conversation has now signed with the customer and gets them to thinking about their disaster recovery and high availability solutions. This issue is also very prominent on SQL Server forums and usually has the title of “Help, my transaction log has filled up my disk” or “Help, my transaction log is many times the size of my database”. In cases where the client only needs the previous full nights backup, I am able to change the recovery model to SIMPLE and shrink the transaction log using DBCC SHRINKFILE (2,1) or by specifying the transaction log file name by using DBCC SHRINKFILE (file_name, target_size). When the client needs point in time recovery then in most cases I will still end up switching the client to the SIMPLE recovery model to truncate the transaction log followed by a full backup. I will then schedule a SQL Agent job to make the regular transaction log backups with an interval determined by the client to meet their service level agreements. It should also be noted that typically when I find an overgrown transaction log the virtual log file count is also out of control. I clean up will always take that into account as well.  That is a subject for a future blog post. If your SQL Server is facing any issue we can Fix Your SQL Server. Additional reading: Monitoring SQL Server Database Transaction Log Space Growth – DBCC SQLPERF(logspace)  SQL SERVER – How to Stop Growing Log File Too Big Shrinking Truncate Log File – Log Full Reference: Pinal Dave (http://blog.sqlauthority.com)Filed under: PostADay, SQL, SQL Authority, SQL Backup and Restore, SQL Query, SQL Server, SQL Tips and Tricks, T SQL

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• SQL SERVER – Preserve Leading Zero While Coping to Excel from SSMS

  - by pinaldave

  Earlier I wrote two articles about how to efficiently copy data from SSMS to Excel. Since I wrote that post there are plenty of interest generated on this subject. There are a few questions I keep on getting over this subject. One of the question is how to get the leading zero preserved while copying the data from SSMS to Excel. Well it is almost the same way as my earlier post SQL SERVER – Excel Losing Decimal Values When Value Pasted from SSMS ResultSet. The key here is in EXCEL and not in SQL Server. The step here is to change the format of Excel Cell to Text from Numbers and that will preserve the value of the with leading or trailing Zeros in Excel. However, I assume this is done for display purpose only because once you convert column to Text you may find it difficult to do numeric operations over the column for example Aggregation, Average etc. If you need to do the same you should either convert the columns back to Numeric in Excel or do the process in Database and export the same value as along with it as well. However, I have seen in requirement in the real world where the user has to have a numeric value with leading Zero values in it for display purpose. Here is my suggestion, instead of manipulating numeric value in the database and converting it to character value the ideal thing to do is to store it as a numeric value only in the database. Whatever changes you want to do for display purpose should be handled at the time of the display using the format function of SQL or Application Language. Honestly, database is data layer and presentation is presentation layer – they are two different things and if possible they should not be mixed. If due to any reason you cannot follow above advise and you need is to have append leading zeros in the database only here are two of my previous articles I suggest you to refer them. I am open to learn new tricks as these articles are almost three years old. Please share your opinion and suggestions in the comments area. SQL SERVER – Pad Ride Side of Number with 0 – Fixed Width Number Display SQL SERVER – UDF – Pad Ride Side of Number with 0 – Fixed Width Number Display Reference: Pinal Dave (http://blog.SQLAuthority.com) Filed under: PostADay, SQL, SQL Authority, SQL Function, SQL Query, SQL Server, SQL Tips and Tricks, T SQL, Technology Tagged: Excel

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• SQLAuthority News – Weekend Experiment with NuoDB – Points to Pondor and Whitepaper

  - by pinaldave

  This weekend I have downloaded the latest beta version of NuoDB. I found it much improved and better UI. I was very much impressed as the installation was very smooth and I was up and running in less than 5 minutes with the product. The tools which are related to the Administration of the NuoDB seems to get makeover during this beta release. As per the claim they support now Solaris platform and have improved the native MacOS installation. I neither have Mac nor Solaris – I wish I would have experimented with the same. I will appreciate if anyone out there can confirm how the installations goes on these platforms. I have previously blogged about my experiment with NuoDB here: SQL SERVER – Weekend Project – Experimenting with ACID Transactions, SQL Compliant, Elastically Scalable Database SQL SERVER – Beginning NuoDB – Who will Benefit and How to Start SQL SERVER – Follow up on Beginning NuoDB – Who will Benefit and How to Start – Part 2 I am very impressed with the product so far and I have decided to understand the product further deep. Here are few of the questions which I am going to try to find answers with regards to NuoDB. Just so it is clear – NuoDB is not NOSQL, matter of the fact, it is following all the ACID properties of the database. If ACID properties are crucial why many NoSQL products are not adhering to it? (There are few out there do follow ACID but not all). I do understand the scalability of the database however does elasticity is crucial for the database and if yes how? (Elasticity is where the workload on the database is heavily fluctuating and the need of more than a single database server is coming up). How NuoDB has built scalable, elastic and 100% ACID compliance database which supports multiple platforms? How is NOSQL compared to NuoDB’s new architecture? In the next coming weeks, I am going to explore above concepts and dive deeper into the understanding of the same. Meanwhile I have read following white paper written by Experts at University of California at Santa Barbara. Very interesting read and great starter on the subject Database Scalability, Elasticity, and Autonomy in the Cloud. Additionally, my questions are also talking about NoSQL, this weekend I have started to learn about NoSQL from Pluralsight‘s online learning library. I will share my experience very soon. Reference: Pinal Dave (http://blog.SQLAuthority.com) Filed under: PostADay, SQL, SQL Authority, SQL Query, SQL Server, SQL Tips and Tricks, SQL White Papers, T SQL, Technology Tagged: NuoDB

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• SQL SERVER – CSVExpress and Quick Data Load

  - by pinaldave

  One of the newest ETL tools is CSVexpress.com.  This is a program that can quickly load any CSV file into ODBC compliant databases uses data integration.  For those of you familiar with databases and how they operate, the question that comes to mind might be what use this program will have in your life. I have written earlier article on this subject over here SQL SERVER – Import CSV into Database – Transferring File Content into a Database Table using CSVexpress. You might know that RDBMS have automatic support for loading CSV files into tables – but it is not quite as easy as one click of a button.  First of all, most databases have a command line interface and you need the file and configuration script in order to load up.  You also need to know enough to write the script – which for novices can be extremely daunting.  On top of all this, if you work with more than one type of RDBMS, you need to know the ins and outs of uploading and writing script for more than one program. So you might begin to see how useful CSVexpress.com might be!  There are many other tools that enable uploading files to a database.  They can be very fancy – some can generate configuration files automatically, others load the data directly.  Again, novices will be able to tell you why these aren’t the most useful programs in the world.  You see, these programs were created with SQL in mind, not for uploading data.  If you don’t have large amounts of data to upload, getting the configurations right can be a long process and you will have to check the code that is generated yourself.  Not exactly “easy to use” for novices. That makes CSVexpress.com one of the best new tools available for everyone – but especially people who don’t want to learn a lot of new material all at once.  CSVexpress has an easy to navigate graphical user interface and no scripting or coding is required.  There are built-in constraints and data validations, and you can configure transforms and reject records right there on the screen.  But the best thing of all – it’s free! That’s right, you can download CSVexpress for free from www.csvexpress.com and start easily uploading and configuring riles almost immediately.  If you’re currently happy with your method of data configuration, keep up with the good work.  For the rest of us, there’s CSVexpress.com. Reference: Pinal Dave (http://blog.SQLAuthority.com) Filed under: Pinal Dave, PostADay, SQL, SQL Authority, SQL Query, SQL Server, SQL Tips and Tricks, SQL Utility, T SQL, Technology

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• SQL SERVER – Difference Between CURRENT_TIMESTAMP and GETDATE() – CURRENT_TIMESTAMP Equivalent in SQL Server

  - by pinaldave

  A common question – I often get from Oracle/MySQL Professionals: “What is the Equivalent to CURRENT_TIMESTAMP in SQL Server?” Here is a common question I often get from SQL Server Professionals: “What are differences between Difference Between CURRENT_TIMESTAMP and GETDATE ()?” Very simple question but have showed up so frequently that I feel like to write about it. Well in SQL Server GETDATE() is Equivalent to CURRENT_TIMESTAMP. However, if you use CURRENT_TIMESTAMP in your select statement it will work fine. You can see in the above example – both of them returns the same value. Now let us go to next question regarding difference between GETDATE and CURRENT_TIMESTAMP. Well, the matter of the fact, there is no difference between them in SQL Server (Reference Link). CURRENT_TIMESTAMP is an ANSI SQL function, whereas GETDATE is T-SQL implementation of the same function. Both of them derive value from the operating system of the computer on which SQL Server instance is running. Above discussion prompts another question – in this case, what should one use GETDATE or CURRENT_TIMESTAMP? Well, this is indeed tricky and interesting question. I think I am very comfortable using the GETDATE () so I will go to use it but a matter of the fact there is no right or wrong answer. If you want to follow ancient saying “When in Rome, do as the Romans do”, I suggest using the GETDATE (), or continue using CURRENT_TIMESTAMP. With that said, there is one very important property we all need to keep in mind. If you use CURRENT_TIMESTAMP while creating an object, they are automatically converted to GETDATE() and stored internally. To illustrate what I am suggesting here is the example - Create a table using the following script CREATE TABLE [dbo].[TestTable]( [Cold2] [datetime] NULL ) ON [PRIMARY] GO ALTER TABLE [dbo].[TestTable] ADD DEFAULT (CURRENT_TIMESTAMP) FOR [Cold2] GO Now go to SSMS and generate the script for the table and you will notice following syntax. CREATE TABLE [dbo].[TestTable]( [Cold2] [datetime] NULL ) ON [PRIMARY] GO ALTER TABLE [dbo].[TestTable] ADD DEFAULT (GETDATE()) FOR [Cold2] GO You can notice that SQL Server have automatically converted CURRENT_TIMESTAMP to GETDATE(). I guess this gives us an idea how they behave. Now go ahead and make your choice! Do let me know which one will you use CURRENT_TIMESTAMP or GETDATE () in the comments area. Reference: Pinal Dave (http://blog.sqlauthority.com) Filed under: PostADay, SQL, SQL Authority, SQL DateTime, SQL Query, SQL Server, SQL Tips and Tricks, T SQL, Technology

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• SQLAuthority News – Updates on Contests, Books and SQL Server

  - by pinaldave

  There are lots of things happening on this blog and I feel sometime it is difficult to keep up. One of the suggestion I keep on receiving if there is a single page where one can visit and know the updates. I did consider of the same at some point but in era of RSS Feed it is difficult to have proper audience to that page. Here are few updates on various contest and books give away in recent time. Combo set of 5 Joes 2 Pros Book – 1 for YOU and 1 for Friend – I have received so many entries for this contest. Many have sent me email asking if this contest can be extended by couple of days. For the same the deadline for this contest is now Nov 10th 7 AM. You can send your entries by that time. The prize is 2 combo set of Joes 2 Pros is of USD 444. If you have not take part in the contest please take part now. Guess What is in the box? – There were many entry for this contest. We played this contest on blog as well, facebook. The answer of this contest was announced in 2 days in blog post announcing my new book. The winner was Manas Dash from Bangalore. He answered “The box will contain SQL book authored by Vinod and Pinal”. This was the closest answer we received. Win 5 SQL Programming Book Contest will have winner announced by Nov 15th and winners will be sent email. Win 5 SQL Wait Stats Book Contest is closed and winners have been sent their award. My third book SQL Server Interview Questions and Answers have run out of stock in India in 36 hours of its launch. We are working very hard to make it available again. Thank you again for excellent support! Without your participation all the give away have no significance. Reference: Pinal Dave (http://blog.SQLAuthority.com) Filed under: About Me, Pinal Dave, PostADay, Readers Contribution, Readers Question, SQL, SQL Authority, SQL Puzzle, SQL Query, SQL Server, SQL Tips and Tricks, T SQL, Technology

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• SQLAuthority News – Presented Soft Skill Session on Presentation Skills at SQL Bangalore on May 3, 2014

  - by Pinal Dave

  I have presented on various database technologies for almost 10 years now. SQL, Database and NoSQL have been part of my life. Earlier this month, I had the opportunity to present on the topic Performing an Effective Presentation. I must say it was blast to prepare as well as present this session. This event was part of the SQL Bangalore community. If you are in Bangalore, you must be part of this group. SQL Bangalore is a wonderful community and we always have a great response when we present on technology. It is SQL User Group and we discuss everything SQL there. This month we had SQL Server 2014 theme and we had a community launch of SQL Server. We have the best of the best speakers presenting on SQL Server 2014 technology. The event had amazing speakers and each of them did justice to the subject. You can read about this over here. In this session I told a story from my life. I talked about who inspired me and how I learned to speak in public. I told stories about two legends  who have inspired me. There is no video recording of this session. If you want to get resources from this session, please sign up my newsletter at http://bit.ly/sqllearn. Well, I had a great time at this event. We had over 250 people showed up at this event and had a grand  time together. I personally enjoyed a session of Amit Benerjee, Balmukund Lakhani and Vinod Kumar. Ken and Surabh also entertained the audience. Overall, this was a grand event and if you were in Bangalore and did not make it to this event. You did miss out on a few things. Here are a few photos of this event. SQL Bangalore UG Nupur, Chandra, Shaivi, Balmukund, Amit, Vinod [captions This] SQL Bangalore UG Audience Pinal Dave presenting at SQL UG in Bangalore Here are few of the slides from this presentation: Reference: Pinal Dave (http://blog.sqlauthority.com)Filed under: PostADay, SQL, SQL Authority, SQL Query, SQL Server, SQL Tips and Tricks, SQLAuthority Author Visit, SQLAuthority News, T SQL

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• SQL SERVER – Quiz and Video – Introduction to SQL Error Actions

  - by pinaldave

  This blog post is inspired from SQL Programming Joes 2 Pros: Programming and Development for Microsoft SQL Server 2008 – SQL Exam Prep Series 70-433 – Volume 4. [Amazon] | [Flipkart] | [Kindle] | [IndiaPlaza] This is follow up blog post of my earlier blog post on the same subject - SQL SERVER – Introduction to SQL Error Actions – A Primer. In the article we discussed various basics terminology of the error handling. The article further covers following important concepts of error handling. Introduction to SQL Error Actions Statement Termination Scope Abortion Batch Termination Above three are the most important concepts related to error handling and SQL Server.  There are many more things one has to learn but without beginners fundamentals one can’t learn the advanced concepts. Let us have small quiz and check how many of you get the fundamentals right. Quiz 1.) Which SQL Server error action happens for errors with a severity of 11-16 when you set the XACT_ABORT setting to ON? You will get Statement Termination. You will get Scope Abortion. You will get Batch Abortion. You will get Connection Termination. SQL Server will pick the error action. 2.) Which SQL Server error action happens for errors with a severity of 11-16 when you set the XACT_ABORT setting to OFF? You will get Statement Termination You will get Scope Abortion You will get Batch Abortion You will get Connection Termination SQL Server will pick the error action Now make sure that you write down all the answers on the piece of paper. Watch following video and read earlier article over here. If you want to change the answer you still have chance. Solution 1) 3 2) 5 Now compare let us check the answers and compare your answers to following answers. I am very confident you will get them correct. Available at USA: Amazon India: Flipkart | IndiaPlaza Volume: 1, 2, 3, 4, 5 Please leave your feedback in the comment area for the quiz and video. Did you know all the answers of the quiz? Reference: Pinal Dave (http://blog.sqlauthority.com) Filed under: Joes 2 Pros, PostADay, SQL, SQL Authority, SQL Query, SQL Server, SQL Tips and Tricks, T SQL, Technology

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• SQL SERVER – Simple Explanation and Puzzle with SOUNDEX Function and DIFFERENCE Function

  - by pinaldave

  Earlier this week I asked a question where I asked how to Swap Values of the column without using CASE Statement. Read here: A Puzzle – Swap Value of Column Without Case Statement,there were more than 50 solutions proposed in the comment. There were many creative solutions. I have mentioned my personal favorite (different ones) here: Solution of Puzzle – Swap Value of Column Without Case Statement. However, I received lots of questions regarding one of the Solution by SIJIN KUMAR V P. He has used the function SOUNDEX in his solution. The request was to explain how SOUNDEX and DIFFERENCE works. Well, there are pretty decent documentations provided over here SOUNDEX function and DIFFERENCE over on MSDN and if I attempt to explain this function I will end up writing the same details which are available on MSDN. Instead of writing theory, we will try to learn this function by using a couple of simple puzzles. You try to solve the puzzles using the MSDN and see if you can learn something very quickly. In simple words - SOUNDEX converts an alphanumeric string to a four-character code to find similar-sounding words or names. The first character of the code is the first character of character_expression and the second through fourth characters of the code are numbers that represent the letters in the expression. Vowels incharacter_expression are ignored unless they are the first letter of the string. DIFFERENCE function returns an integer value. The  integer returned is the number of characters in the SOUNDEX values that are the same. The return value ranges from 0 through 4: 0 indicates weak or no similarity, and 4 indicates strong similarity or the same values. Learning Puzzle 1: Now let us run following four queries and observe its output. SELECT SOUNDEX('SQLAuthority') SdxValue SELECT SOUNDEX('SLTR') SdxValue SELECT SOUNDEX('SaLaTaRa') SdxValue SELECT SOUNDEX('SaLaTaRaM') SdxValue When you look at the result set all the four values are same. The reason for all the values to be same is as for SQL Server SOUNDEX function all the four strings are similarly sounding string. Learning Puzzle 2: Now let us run following five queries and observe its output. SELECT DIFFERENCE (SOUNDEX('SLTR'),SOUNDEX('SQLAuthority')) SELECT DIFFERENCE (SOUNDEX('TH'),SOUNDEX('SQLAuthority')) SELECT DIFFERENCE ('SQLAuthority',SOUNDEX('SQLAuthority')) SELECT DIFFERENCE ('SLTR',SOUNDEX('SQLAuthority')) SELECT DIFFERENCE ('SLTR','SQLAuthority') When you look at the result set you will get the result in the ranges from 1 to 4. Here is how it works if your result is 0 which means absolutely not relevant to each other and if your result is 1 which means the results are relevant to each other. Have you ever used above two functions in your business need or on production server? If yes, would you please leave a comment with use cases. I believe it will be beneficial to everyone. Reference: Pinal Dave (http://blog.SQLAuthority.com) Filed under: PostADay, SQL, SQL Authority, SQL Puzzle, SQL Query, SQL Server, SQL Tips and Tricks, T SQL, Technology

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• SQL SERVER – Creating All New Database with Full Recovery Model

  - by pinaldave

  Sometimes, complex problems have very simple solutions. Let us see the following email which I received recently. “Hi Pinal, In our system when we create new database, by default, they are all created with the Simple Recovery Model. We have to manually change the recovery model after we create the database. We used the following simple T-SQL code: CREATE DATABASE dbname. We are very frustrated with this situation. We want all our databases to have the Full Recovery Model option by default. We are considering the following methods; please suggest the most efficient one among them. 1) Creating a Policy; when it is violated, the database model can be fixed 2) Triggers at Server Level 3) Automated Job which goes through all the databases and checks their recovery model; if the DBA has not changed the model, then the job will list the Databases and change their recovery model Also, we have a situation where we need a database in the Simple Recovery Model as well – how to white list them? Please suggest the best method.” Indeed, an interesting email! The answer to their question, i.e., which is the best method to fit their needs (white list, default, etc)? It will be NONE of the above. Here is the solution in one line and also the easiest way: Just go to your Model database: Path in SSMS >> Databases > System Databases >> model >> Right Click Properties >> Options >> Recovery Model - Select Full from dropdown. Every newly created database takes its base template from the Model Database. If you create a custom SP in the Model Database, when you create a new database, it will automatically exist in that database. Any database that was already created before making changes in the Model Database will not be affected at all. Creating Policy is also a good method, and I will blog about this in a separate blog post, but looking at current specifications of the reader, I think the Model Database should be modified to have a Full Recovery Option. While writing this blog post, I remembered my another blog post where the model database log file was growing drastically even though there were no transactions SQL SERVER – Log File Growing for Model Database – model Database Log File Grew Too Big. NOTE: Please do not touch the Model Database unnecessary. It is a strict “No.” If you want to create an object that you need in all the databases, then instead of creating it in model database, I suggest that you create a new database called maintenance and create the object there. Reference: Pinal Dave (http://blog.SQLAuthority.com) Filed under: PostADay, Readers Question, SQL, SQL Authority, SQL Query, SQL Server, SQL Tips and Tricks, T SQL, Technology

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• SQL Contests – Solution – Identify the Database Celebrity

  - by Pinal Dave

  Last week we were running contest Identify the Database Celebrity and we had received a fantastic response to the contest. Thank you to the kind folks at NuoDB as they had offered two USD 100 Amazon Gift Cards to the winners of the contest. We had also additional contest that users have to download and install NuoDB and identified the sample database. You can read about the contest over here. Here is the answer to the questions which we had asked earlier in the contest. Part 1: Identify Database Celebrity Personality 1 – Edgar Frank “Ted” Codd (August 19, 1923 – April 18, 2003) was an English computer scientist who, while working for IBM, invented the relational model for database management, the theoretical basis for relational databases. He made other valuable contributions to computer science, but the relational model, a very influential general theory of data management, remains his most mentioned achievement. (Wki) Personality 2 – James Nicholas “Jim” Gray (born January 12, 1944; lost at sea January 28, 2007; declared deceased May 16, 2012) was an American computer scientist who received the Turing Award in 1998 “for seminal contributions to database and transaction processing research and technical leadership in system implementation.” (Wiki) Personality 3 – Jim Starkey (born January 6, 1949 in Illinois) is a database architect responsible for developing InterBase, the first relational database to support multi-versioning, the blob column type, type event alerts, arrays and triggers. Starkey is the founder of several companies, including the web application development and database tool company Netfrastructure and NuoDB. (Wiki) Part 2: Identify NuoDB Samples Database Names In this part of the contest one has to Download NuoDB and install the sample database Hockey. Hockey is sample database and contains few tables. Users have to install sample database and inform the name of the sample databases. Here is the valid answer. HOCKEY PLAYERS SCORING TEAM Once again, it was indeed fun to run this contest. I have received great feedback about it and lots of people wants me to run similar contest in future. I promise to run similar interesting contests in the near future. Winners Within next two days, we will let winners send emails. Winners will have to confirm their email address and NuoDB team will send them directly Amazon Cards. Once again it was indeed fun to run this contest. Reference: Pinal Dave (http://blog.SQLAuthority.com) Filed under: PostADay, SQL, SQL Authority, SQL Query, SQL Server, SQL Tips and Tricks, T SQL

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• SQL SERVER – Working with FileTables in SQL Server 2012 – Part 2 – Methods to Insert Data Into Table

  - by pinaldave

  Read Part 1 Working with FileTables in SQL Server 2012 – Part 1 – Setting Up Environment In this second part of the series, we will see how we can insert the files into the FileTables. There are two methods to insert the data into FileTables: Method 1: Copy Paste data into the FileTables folder First, find the folder where FileTable will be storing the files. Go to Databases >> Newly Created Database (FileTableDB) >> Expand Tables. Here you will see a new folder which says “FileTables”. When expanded, it gives the name of the newly created “FileTableTb”. Right click on the newly created table, and click on “Explore FileTable Directory”. This will open up the folder where the FileTable data will be stored. When clicked on the option it will open up following folder in my local machine where the FileTable data will be stored. \\127.0.0.1\mssqlserver\FileTableDB\FileTableTb_Dir You can just copy your document just there. I copied few word document there and ran select statement to see the result. USE [FileTableDB] GO SELECT * FROM FileTableTb GO SELECT * returns all the rows. Here is SELECT statement which has only few columns selected from FileTable. SELECT [name] ,[file_type] ,CAST([file_stream] AS VARCHAR) FileContent ,[cached_file_size] ,[creation_time] ,[last_write_time] ,[last_access_time] FROM [dbo].[FileTableTb] GO I believe this is the simplest method to populate FileTable, because you just have to move the files to the specific table. Method 2: T-SQL Insert Statement There are always cases when you might want to programmatically insert the images into SQL Server File table. Here is a quick method which you can use to insert the data in the file table. I have inserted a very small text file using T-SQL, and later on, reading that using SELECT statement demonstrated in method 1 above. INSERT INTO [dbo].[FileTableTb] ([name],[file_stream]) SELECT 'NewFile.txt', * FROM OPENROWSET(BULK N'd:\NewFile.txt', SINGLE_BLOB) AS FileData GO The above T-SQL statement will copy the NewFile.txt to new location. When you run SELECT statement, it will retrieve the file and list in the resultset. Additionally, it returns the content in the SELECT statement as well. I think it is a pretty interesting way to insert the data into the FileTable. SELECT [name] ,[file_type] ,CAST([file_stream] AS VARCHAR) FileContent ,[cached_file_size] ,[creation_time] ,[last_write_time] ,[last_access_time] FROM [dbo].[FileTableTb] GO There are more to FileTable and we will see those in my future blog posts. Reference: Pinal Dave (http://blog.sqlauthority.com) Filed under: PostADay, SQL, SQL Authority, SQL Query, SQL Server, SQL Tips and Tricks, T SQL, Technology Tagged: Filestream

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• SQLAuthority News – 5th Anniversary Giveaways

  - by pinaldave

  Please read my 5th Anniversary post and my quick note on history of the Database. I am sure that we all have friends and we value friendship more than anything. In fact, the complete model of Facebook is built on friends. If you have lots of friends, you must be a lucky person. Having a lot of friends is indeed a good thing. I consider all you blog readers as my friends so now I want do something for you. What is it? Well, send me details about how many of your friends like my page and you would have a chance to win lots of learning materials for yourself and your friends. Here are the exciting prizes awaiting the lucky winner: Combo set of 5 Joes 2 Pros Book – 1 for YOU and 1 for Friend This is USD 444 (each set USD 222) worth gift. It contains all the five Joes 2 Pros books (Vol1, Vol2, Vol3, Vol4, Vol5) + 1 Learning DVD. [Amazon] | [Flipkart] If in case you submitted an entry but didn’t win the Combo set of 5 Joes 2 Pros books, you could still will  my SQL Server Wait Stats book as a consolation prize! I will pick the next 5 participants who have the highest number of friends who “liked” the Facebook page, http://facebook.com/SQLAuth. Instead of sending one copy, I will send you 2 copies so you can share one copy with a friend of yours. Well, it is important to share our learning and love with friends, isn’t it? Note: Just take a screenshot of http://facebook.com/SQLAuth using Print Screen function and send it by Nov 7th to pinal ‘at’ sqlauthority.com.. There are no special freebies to early birds so take your time and see if you can increase your friends like count by Nov 7th. Guess – What is in it? It is quite possible you are not a Facebook or Twitter user. In that case you can still win a surprise from me. You have 2 days to guess what is in this box. If you guess it correct and you are one of the first 5 persons to have the correct answer – you will get what is in this box for free. Please note that you have only 48 hours to guess. Please give me your guess by commenting to this blog post. Reference:  Pinal Dave (http://blog.SQLAuthority.com) Filed under: About Me, Pinal Dave, PostADay, Readers Contribution, Readers Question, SQL, SQL Authority, SQL Milestone, SQL Query, SQL Server, SQL Tips and Tricks, SQLAuthority News, SQLServer, T SQL, Technology

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• SQLAuthority News – SQL Server Wait Stats – eBook to Download on Kindle – Answer to FREE PDF Download Request

  - by pinaldave

  Being a book author is a completely new experience for me. I am yet to come across the issues faced by expert book authors. I assume that these interesting issues can be routine ones for expert book authors. One of the biggest requests I am getting for my SQL Server Wait Stats [Amazon] | [Flipkart] | [Kindle] book is my humble attempt to write a book. This is our very first experiment, and the book is beginning of the subject of SQL Server Wait Stats; we will come up with a new version of the book later next year when we have enough information for the SQL Server 2012 version. Following are the top 2 requests that I keep on receiving in emails, on blogs, Twitter, and Facebook. “Please send us FREE PDF of your book so we do not have to purchase it.” “If you can share with us the eBook (free and downloadable) format of your book, we will share it with everybody we know and you will get additional exposure.” Here is my response for the abovementioned requests: If you really need my book and cannot purchase it due to financial trouble, then feel free to let me know and I will purchase it myself and ship it to you. If you are in a country where the print book not available, then you can buy the Kindle book, which is available online in any country, and you can just read it on your computer and mobile devices. You DO NOT have to own a Kindle to read a Kindle format book. You can freely download Kindle software on your desired format and purchase the book online. For next 5 days, the kindle book is available at 3.99 in USA, and in other countries, the price is anywhere between 3.99 and 5.99. The price will go up by USD 2 everywhere across the world after 1st November, 2011. Here is the link to download Kindle Software for free PC, WP7, and in marketplace for various other mobile devices. I thank you for giving warm response to SQL Server Wait Stats book. I am motivated to write the next expanded version of this book. Reference: Pinal Dave (http://blog.SQLAuthority.com) Filed under: About Me, Database, Pinal Dave, PostADay, SQL, SQL Authority, SQL Query, SQL Server, SQL Tips and Tricks, T SQL, Technology

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• SQLAuthority News – Presenting at Great Indian Developer Summit 2012 – SQL Server Misconception and Resolutions

  - by pinaldave

  Earlier during TechEd 2012, I presented a session on SQL Server Misconception and Resolutions. It was a pleasure to present this session with Vinod Kumar during the event. Great Indian Developer Summit is around the corner and I will be presenting there once again with the same topic. We had an excellent response during the last event; the hall was so filled, but there were plenty who were not able to get into the session as there was no place for them to sit or stand inside. Well, here is another chance for all who missed the presentation. New Additions During the last session, we were a two-presenter tag team, and we presented the session in a sense that it would suit two speakers in one stage. But this time, I am the only presenter, so I decided to present this session in a much different way. I will still assume there are two presenters. One of the presenters will be me, of course, and the second person will be YOU! Yes, you read that right – you will be presenting this session with me. If you wonder how, well, you will have to attend the session to figure it out. Talking Points We will be talking about the following topics in the session which we will relate to SQL Server: Moon Landing Napoléon Bonaparte Wall of China Bollywood …and of course, SQL Server itself. I promise that this 45 minute- presentation will be the one of the highlights of the event for you. Goodies I can only promise 20 goodies as of the moment. I might bring more when you meet me there. Session Details Title: SQL Server Misconceptions and Resolution – A Practical Perspective (Add to Calendar) Abstract: “The earth is flat”! – An ancient common misconception, which has been proven incorrect as we progressed in modern times. In this session, we will see various database misconceptions prevailing and their resolutions with the aid of the demos. In this unique session, the audience will be a part of the conversation and resolution. Date and Time: April 17, 2012, 16:55 to 17:40 Location: J. N. Tata Auditorium, National Science Symposium Complex (NSSC), Bangalore, India Add to Calendar Reference : Pinal Dave (http://blog.sqlauthority.com) Filed under: PostADay, SQL, SQL Authority, SQL Query, SQL Server, SQL Tips and Tricks, SQLAuthority Author Visit, SQLAuthority News, T SQL, Technology

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• SQL SERVER – How to easily work with Database Diagrams

  - by Pinal Dave

  Databases are very widely used in the modern world. Regardless of the complexity of a database, each one requires in depth designing. To practice along please Download dbForge Studio now.  The right methodology of designing a database is based on the foundations of data normalization, according to which we should first define database’s key elements – entities. Afterwards the attributes of entities and relations between them are determined. There is a strong opinion that the process of database designing should start with a pencil and a blank sheet of paper. This might look old-fashioned nowadays, because SQL Server provides a much wider functionality for designing databases – Database Diagrams. When using SSMS for working with Database Diagrams I realized two things – on the one hand, visualization of a scheme allows designing a database more efficiently; on the other – when it came to creating a big scheme, some difficulties occurred when designing with SSMS. The alternatives haven’t taken long to wait and dbForge Studio for SQL Server is one of them. Its functions offer more advantages for working with Database Diagrams. For example, unlike SSMS, dbForge Studio supports an opportunity to drag-and-drop several tables at once from the Database Explorer. This is my opinion but personally I find this option very useful. Another great thing is that a diagram can be saved as both a graphic file and a special XML file, which in case of identical environment can be easily opened on the other server for continuing the work. During working with dbForge Studio it turned out that it offers a wide set of elements to operate with on the diagram. Noteworthy among such elements are containers which allow aggregating diagram objects into thematic groups. Moreover, you can even place an image directly on the diagram if the scheme design is based on a standard template. Each of the development environments has a different approach to storing a diagram (for example, SSMS stores them on a server-side, whereas dbForge Studio – in a local file). I haven’t found yet an ability to convert existing diagrams from SSMS to dbForge Studio. However I hope Devart developers will implement this feature in one of the following releases. All in all, editing Database Diagrams through dbForge Studio was a nice experience and allowed speeding-up the common database designing tasks. Download dbForge Studio now. Reference: Pinal Dave (http://blog.sqlauthority.com) Filed under: PostADay, SQL, SQL Authority, SQL Query, SQL Server, SQL Tips and Tricks, SQL Utility, T SQL

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• SQL SERVER – Concurrancy Problems and their Relationship with Isolation Level

  - by pinaldave

  Concurrency is simply put capability of the machine to support two or more transactions working with the same data at the same time. This usually comes up with data is being modified, as during the retrieval of the data this is not the issue. Most of the concurrency problems can be avoided by SQL Locks. There are four types of concurrency problems visible in the normal programming. 1)      Lost Update – This problem occurs when there are two transactions involved and both are unaware of each other. The transaction which occurs later overwrites the transactions created by the earlier update. 2)      Dirty Reads – This problem occurs when a transactions selects data that isn’t committed by another transaction leading to read the data which may not exists when transactions are over. Example: Transaction 1 changes the row. Transaction 2 changes the row. Transaction 1 rolls back the changes. Transaction 2 has selected the row which does not exist. 3)      Nonrepeatable Reads – This problem occurs when two SELECT statements of the same data results in different values because another transactions has updated the data between the two SELECT statements. Example: Transaction 1 selects a row, which is later on updated by Transaction 2. When Transaction A later on selects the row it gets different value. 4)      Phantom Reads – This problem occurs when UPDATE/DELETE is happening on one set of data and INSERT/UPDATE is happening on the same set of data leading inconsistent data in earlier transaction when both the transactions are over. Example: Transaction 1 is deleting 10 rows which are marked as deleting rows, during the same time Transaction 2 inserts row marked as deleted. When Transaction 1 is done deleting rows, there will be still rows marked to be deleted. When two or more transactions are updating the data, concurrency is the biggest issue. I commonly see people toying around with isolation level or locking hints (e.g. NOLOCK) etc, which can very well compromise your data integrity leading to much larger issue in future. Here is the quick mapping of the isolation level with concurrency problems: Isolation Dirty Reads Lost Update Nonrepeatable Reads Phantom Reads Read Uncommitted Yes Yes Yes Yes Read Committed No Yes Yes Yes Repeatable Read No No No Yes Snapshot No No No No Serializable No No No No I hope this 400 word small article gives some quick understanding on concurrency issues and their relation to isolation level. Reference: Pinal Dave (http://blog.SQLAuthority.com) Filed under: Pinal Dave, PostADay, SQL, SQL Authority, SQL Query, SQL Server, SQL Tips and Tricks, T SQL, Technology

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