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  • How do I use Linq-to-sql to iterate db records?

    - by Seth Spearman
    I asked on SO a few days ago what was the simplest quickest way to build a wrapper around a recently completed database. I took the advice and used sqlmetal to build linq classes around my database design. Now I am having two problems. One, I don't know LINQ. And, two, I have been shocked to realize how hard it is to learn. I have a book on LINQ (Linq In Action by Manning) and it has helped some but at the end of the day it is going to take me a couple of weeks to get traction and I need to make some progress on my project today. So, I am looking for some help getting started. Click HERE To see my simple database schema. Click HERE to see the vb class that was generated for the schema. My needs are simple. I have a console app. The main table is the SupplyModel table. Most of the other tables are child tables of the SupplyModel table. I want to iterate through each of Supply Model records. I want to grab the data for a supply model and then DoStuff with the data. And I also need to iterate through the child records, for each supply model, for example the NumberedInventories and DoStuff with that as well. I need help doing this in VB rather than C# if possible. I am not looking for the whole solution...if you can supply a couple of code-snippets to get me on my way that would be great. Thanks for your help. Seth

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  • MySQL-python 1.2.3 and OS X 10.5: 64- or 32-bit?

    - by Dave Everitt
    I've been happily using Django and MySQL in development on an existing machine running OS X 10.4 Tiger, and have set up a similar environment in 10.5 Leopard on a new 64-bit MacBook, with a working MySQL and Python 2.6.4. However, now I want them to communicate, easy_install MySQL-python gave ld warnings that the file is not of the required architecture, which led me to test my Python 2.4.6 install (from the Mac OS X disc image): >>> import sys >>> sys.maxint 2147483647 Ah. So my Python install appears to be 32-bit and (I think?) won't install MySQL-python for my 64-bit MySQL. There are lots of hacks out there for MySQL-python on OS X (mostly 1.2.2), but - after hours of reading - I'm pretty sure they won't fix this architecture mismatch. So I'm stuck because I can't decide whether to: give up, remove the 64-bit MySQL install (thorough methods, please?) and use the 32-bit MySQL disc image instead; re-install Python in 64-bit mode from the tarball, --with-universal archs-64-bit and --enable-universalsdk= as detailed in Python.org's 2.6 news. So my questions for anyone who has encountered this issue are: Is installing 64-bit Python on OS X 10.5 worth bothering with? If so, (naive, lazy question!) how are the two required arguments combined? If I just skip along in 32-bit (as on my working setup) what am I missing? I'm after a hassle-free install that's easy to reproduce on other machines (possible student use) so I'd really welcome your opinions, please!

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  • Can't run install MySQL gem Fedora 14, even after installing mysql-devel, ruby-devel, and rubygems

    - by jonderry
    I'm trying to install the mysql gem via sudo gem install mysql --version 2.7 However, I get the following error: Building native extensions. This could take a while... ........... ERROR: Error installing mysql: ERROR: Failed to build gem native extension. /usr/bin/ruby extconf.rb checking for mysql_query() in -lmysqlclient... no checking for main() in -lm... yes checking for mysql_query() in -lmysqlclient... no checking for main() in -lz... yes checking for mysql_query() in -lmysqlclient... no checking for main() in -lsocket... no checking for mysql_query() in -lmysqlclient... no checking for main() in -lnsl... yes checking for mysql_query() in -lmysqlclient... no *** extconf.rb failed *** Could not create Makefile due to some reason, probably lack of necessary libraries and/or headers. Check the mkmf.log file for more details. You may need configuration options. Any ideas?

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  • Entity Framework LINQ Query using Custom C# Class Method - Once yes, once no - because executing on the client or in SQL?

    - by BrooklynDev
    I have two Entity Framework 4 Linq queries I wrote that make use of a custom class method, one works and one does not: The custom method is: public static DateTime GetLastReadToDate(string fbaUsername, Discussion discussion) { return (discussion.DiscussionUserReads.Where(dur => dur.User.aspnet_User.UserName == fbaUsername).FirstOrDefault() ?? new DiscussionUserRead { ReadToDate = DateTime.Now.AddYears(-99) }).ReadToDate; } The linq query that works calls a from after a from, the equivalent of SelectMany(): from g in oc.Users.Where(u => u.aspnet_User.UserName == fbaUsername).First().Groups from d in g.Discussions select new { UnReadPostCount = d.Posts.Where(p => p.CreatedDate > DiscussionRepository.GetLastReadToDate(fbaUsername, p.Discussion)).Count() }; The query that does not work is more like a regular select: from d in oc.Discussions where d.Group.Name == "Student" select new { UnReadPostCount = d.Posts.Where(p => p.CreatedDate > DiscussionRepository.GetLastReadToDate(fbaUsername, p.Discussion)).Count(), }; The error I get is: LINQ to Entities does not recognize the method 'System.DateTime GetLastReadToDate(System.String, Discussion)' method, and this method cannot be translated into a store expression. My question is, why am I able to use my custom GetLastReadToDate() method in the first query and not the second? I suppose this has something to do with what gets executed on the db server and what gets executed on the client? These queries seem to use the GetLastReadToDate() method so similarly though, I'm wondering why would work for the first and not the second, and most importantly if there's a way to factor common query syntax like what's in the GetLastReadToDate() method into a separate location to be reused in several different places LINQ queries. Please note all these queries are sharing the same object context.

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  • Problem converting MsSql to MySql Stored procedure

    - by karthik
    Original source of MsSql SP is here.. http://www.codeproject.com/KB/database/InsertGeneratorPack.aspx I am using the below MySql stored procedure, created by SQLWAYS [Tool to convert MsSql to MySql]. The purpose of this is to take backup of selected tables to a script file. when the SP returns a value {Insert statements}. When i Execute the Below SP, i am getting a weird Result Set : SQLWAYS_EVAL# ll(cast(UidSQLWAYS_EVAL# 0)),'0')+''','+SQLWAYS_EVAL# ll(UserNameSQLWAYS_EVAL# '+SQLWAYS_EVAL# ll(PasswordSQLWAYS_EVAL# '+ I see a lot of "SQLWAYS_EVAL#" in the code, which is produced in the result too. What values need to be passed instead of "SQLWAYS_EVAL#". So that i get the proper Insert statements for each record in the table. I am new to MySql. Please help me. Its Urgent. Thanks. DELIMITER $$ DROP PROCEDURE IF EXISTS `InsertGenerator` $$ CREATE DEFINER=`root`@`localhost` PROCEDURE `InsertGenerator`() SWL_return: BEGIN -- SQLWAYS_EVAL# to retrieve column specific information -- SQLWAYS_EVAL# table DECLARE v_string VARCHAR(3000); -- SQLWAYS_EVAL# first half -- SQLWAYS_EVAL# tement DECLARE v_stringData VARCHAR(3000); -- SQLWAYS_EVAL# data -- SQLWAYS_EVAL# statement DECLARE v_dataType VARCHAR(1000); -- SQLWAYS_EVAL# -- SQLWAYS_EVAL# columns DECLARE v_colName VARCHAR(50); DECLARE NO_DATA INT DEFAULT 0; DECLARE cursCol CURSOR FOR SELECT column_name,data_type FROM information_schema.`columns` -- WHERE table_name = v_tableName; WHERE table_name = 'tbl_users'; DECLARE CONTINUE HANDLER FOR SQLEXCEPTION BEGIN SET NO_DATA = -2; END; DECLARE CONTINUE HANDLER FOR NOT FOUND SET NO_DATA = -1; OPEN cursCol; SET v_string = CONCAT('INSERT ',v_tableName,'('); SET v_stringData = ''; SET NO_DATA = 0; FETCH cursCol INTO v_colName,v_dataType; IF NO_DATA <> 0 then -- NOT SUPPORTED print CONCAT('Table ',@tableName, ' not found, processing skipped.') close cursCol; LEAVE SWL_return; end if; WHILE NO_DATA = 0 DO IF v_dataType in('varchar','char','nchar','nvarchar') then SET v_stringData = CONCAT(v_stringData,'SQLWAYS_EVAL# ll(',v_colName,'SQLWAYS_EVAL# ''+'); ELSE if v_dataType in('text','ntext') then -- SQLWAYS_EVAL# -- SQLWAYS_EVAL# else SET v_stringData = CONCAT(v_stringData,'SQLWAYS_EVAL# ll(cast(',v_colName,'SQLWAYS_EVAL# 00)),'''')+'''''',''+'); ELSE IF v_dataType = 'money' then -- SQLWAYS_EVAL# doesn't get converted -- SQLWAYS_EVAL# implicitly SET v_stringData = CONCAT(v_stringData,'SQLWAYS_EVAL# y,''''''+ isnull(cast(',v_colName,'SQLWAYS_EVAL# 0)),''0.0000'')+''''''),''+'); ELSE IF v_dataType = 'datetime' then SET v_stringData = CONCAT(v_stringData,'SQLWAYS_EVAL# time,''''''+ isnull(cast(',v_colName, 'SQLWAYS_EVAL# 0)),''0'')+''''''),''+'); ELSE IF v_dataType = 'image' then SET v_stringData = CONCAT(v_stringData,'SQLWAYS_EVAL# ll(cast(convert(varbinary,',v_colName, 'SQLWAYS_EVAL# 6)),''0'')+'''''',''+'); ELSE SET v_stringData = CONCAT(v_stringData,'SQLWAYS_EVAL# ll(cast(',v_colName,'SQLWAYS_EVAL# 0)),''0'')+'''''',''+'); end if; end if; end if; end if; end if; SET v_string = CONCAT(v_string,v_colName,','); SET NO_DATA = 0; FETCH cursCol INTO v_colName,v_dataType; END WHILE; select v_stringData; END $$ DELIMITER ;

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  • Use LINQ, to Sort and Filter items in a List<ReturnItem> collection, based on the values within a Li

    - by Daniel McPherson
    This is tricky to explain. We have a DataTable that contains a user configurable selection of columns, which are not known at compile time. Every column in the DataTable is of type String. We need to convert this DataTable into a strongly typed Collection of "ReturnItem" objects so that we can then sort and filter using LINQ for use in our application. We have made some progress as follows: We started with the basic DataTable. We then process the DataTable, creating a new "ReturnItem" object for each row This "ReturnItem" object has just two properties: ID ( string ) and Columns( List(object) ). The properties collection contains one entry for each column, representing a single DataRow. Each property is made Strongly Typed (int, string, datetime, etc). For example it would add a new "DateTime" object to the "ReturnItem" Columns List containing the value of the "Created" Datatable Column. The result is a List(ReturnItem) that we would then like to be able to Sort and Filter using LINQ based on the value in one of the properties, for example, sort on "Created" date. We have been using the LINQ Dynamic Query Library, which gets us so far, but it doesn't look like the way forward because we are using it over a List Collection of objects. Basically, my question boils down to: How can I use LINQ, to Sort and Filter items in a List(ReturnItem) collection, based on the values within a List(object) property which is part of the ReturnItem class?

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  • Election 2012: Twitter Breaks Records with MySQL

    - by Bertrand Matthelié
    Twitter VP of Infrastructure Operations Engineering Mazen Rawashdeh shared news and numbers yesterday on his blog: "Last night, the world tuned in to Twitter to share the election results as U.S. voters chose a president and settled many other campaigns. Throughout the day, people sent more than 31 million election-related Tweets (which contained certain key terms and relevant hashtags). And as results rolled in, we tracked the surge in election-related Tweets at 327,452 Tweets per minute (TPM). These numbers reflect the largest election-related Twitter conversation during our 6 years of existence, though they don’t capture the total volume of all Tweets yesterday." "Last night, Twitter averaged about 9,965 TPS from 8:11pm to 9:11pm PT, with a one-second peak of 15,107 TPS at 8:20pm PT and a one-minute peak of 874,560 TPM. Seeing a sustained peak over the course of an entire event is a change from the way people have previously turned to Twitter during live events. Now, rather than brief spikes, we are seeing sustained peaks for hours." Congrats to Jeremy Cole, Davi Arnaut and the rest of the team at Twitter for their excellent work! Jeremy recently held a keynote presentation at MySQL Connect describing how MySQL powers Twitter, and why they chose and continue to rely on MySQL for their operations. You can watch the presentation here. He also went into more details during another presentation later that day and you can access the slides here. Below a couple of tweets from Jeremy after what have surely been hectic days...  Keep up the good work guys!

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  • Honing Performance Tuning Skills on MySQL

    - by Antoinette O'Sullivan
    Get hands-on experience with techniques for tuning a MySQL Server with the Authorized MySQL Performance Tuning course.  This course is designed for database administrators, database developers and system administrators who are responsible for managing, optimizing, and tuning a MySQL Server. You can follow this live instructor led training: From your desk. Choose from among the 800+ events on the live-virtual training schedule. In a classroom. A selection of events/locations listed below  Location  Date  Delivery Language  Prague, Czech Republic  1 October 2012  Czech  Warsaw, Poland  9 July 2012  Polish  London, UK  19 November 2012  English  Rome, Italy  23 October 2012  Italian  Lisbon, Portugal  17 September 2012  European Portugese  Aix-en-Provence, France  4 September 2012  French  Strasbourg, France  16 October 2012  French  Nieuwegein, Netherlands  3 September 2012  Dutch  Madrid, Spain  6 August 2012  Spanish  Mechelen, Belgium  1 October 2012  English  Riga, Latvia  10 December 2012  Latvian  Petaling Jaya, Malaysia  10 September 2012  English  Edmonton, Canada  27 August 2012  English  Vancouver, Canada  27 August 2012  English  Ottawa, Canada  26 November 2012  English  Toronto, Canada  26 November 2012  English  Montreal, Canada  26 November 2012  English  Mexico City, Mexico  9 July 2012  Spanish  Sao Paulo, Brazil  2 July 2012  Brazilian Portugese To find a virtual or in-class event that suits you, go or http://oracle.com/education and choose a course and delivery type in your location.  

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  • LINQ to SQL exception: System.OutOfMemoryException

    - by Adam
    Not sure why I keep getting an OutOfMemory exception. I'm using ASP.NET MVC with LINQ to SQL. Here's some of the stack trace: [OutOfMemoryException: Exception of type 'System.OutOfMemoryException' was thrown.] System.Runtime.CompilerServices.RuntimeHelpers._CompileMethod(IntPtr method) +0 System.Reflection.Emit.DynamicMethod.CreateDelegate(Type delegateType) +7652553 System.Data.Linq.SqlClient.ObjectReaderCompiler.Compile(SqlExpression expression, Type elementType) +442 System.Data.Linq.SqlClient.SqlProvider.GetReaderFactory(SqlNode node, Type elemType) +100 System.Data.Linq.SqlClient.SqlProvider.System.Data.Linq.Provider.IProvider.Execute(Expression query) +253 System.Data.Linq.Table1.System.Linq.IQueryProvider.Execute(Expression expression) +49 System.Linq.Queryable.Single(IQueryable1 source, Expression`1 predicate) +301 WorkGrabber.Web.Models.WorkGrabberDataContext.GetJob(Int32 id) +233 WorkGrabber.Web.Controllers.BidsController.New(Int32 jobId) +19

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  • What's the next big thing after LINQ?

    - by Leniel Macaferi
    I started using LINQ (Language Integrated Query) when it was still in beta, more specifically Microsoft .NET LINQ Preview (May 2006). Almost 4 years have passed and here we are using LINQ in a lot of projects for the most diverse tasks. I even wrote my final college project based on LINQ. You see how I like it. LINQ and more recently PLINQ (Parallel LINQ) give our jobs a great boost when it comes to more programming power and less lines of code leading us to more expressive and readable code. I keep thinking what could be the next big language improvement for C# after LINQ. I know there are some promissing language features coming as Code Contracts, etc, but nothing having the impact that LINQ had. What do you think could be the next big thing?

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  • LINQ – TakeWhile and SkipWhile methods

    - by nmarun
    I happened to read about these methods on Vikram's blog and tried testing it. Somehow when I saw the output, things did not seem to add up right. I’m writing this blog to show the actual workings of these methods. Let’s take the same example as showing in Vikram’s blog and I’ll build around it. 1: int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 }; 2:  3: foreach(var number in numbers.TakeWhile(n => n < 7)) 4: { 5: Console.WriteLine(number); 6: } Now, the way I (incorrectly) read the upper bound condition in the foreach loop was: ‘Give me all numbers that pass the condition of n<7’. So I was expecting the answer to be: 5, 4, 1, 3, 2, 0. But when I run the application, I see only: 5, 4, 1,3. Turns out I was wrong (happens at least once a day). The documentation on the method says ‘Returns elements from a sequence as long as a specified condition is true. To show in code, my interpretation was the below code’: 1: foreach (var number in numbers) 2: { 3: if (number < 7) 4: { 5: Console.WriteLine(number); 6: } 7: } But the actual implementation is: 1: foreach(var number in numbers) 2: { 3: if(number < 7) 4: { 5: Console.WriteLine(number); 6: break; 7: } 8: } So there it is, another situation where one simple word makes a difference of a whole world. The SkipWhile method has been implemented in a similar way – ‘Bypasses elements in a sequence as long as a specified condition is true and then returns the remaining elements’ and not ‘Bypasses elements in a sequence where a specified condition is true and then returns the remaining elements’. (Subtle.. very very subtle). It’s feels strange saying this, but hope very few require to read this article to understand these methods.

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  • LINQ for SQL Developers and DBA’s

    - by AtulThakor
    Firstly I’d just like to thank the guys who organise the SQL Server User Group (Martin/Tony/Chris) and for giving me the opportunity to speak at the recent event. Sorry about the slides taking so long but here they are along with some extra information. Firstly the demo’s were all done using LINQPad 4.0 which can be downloaded here: http://www.linqpad.net/ There are 2 versions 3.5/4.0 With 3.5 you should be able to replicate the problem I showed where a query using a parameter which is X characters long would create a different execution plan to a query which uses a parameter which is Y characters long, otherwise I would just use 4.0 The sample database used is AdventureWorksLT2008 which can be downloaded from here: http://msftdbprodsamples.codeplex.com/releases/view/37109 The scripts have been named so that you can select the appropriate way to run them i.e.: C# expression / C#statement, each script can be run individually be highlighting the query and clicking the play symbol or hitting F5. Scripts and Slides: http://sqlblogcasts.com/blogs/atulthakor/An%20Introduction%20to%20LINQ.zip Please don't hesitate in sending any questions via email/twitter, I’ll try my best to answer your questions! Thanks, Atul

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  • Grouping data in LINQ with the help of group keyword

    - by vik20000in
    While working with any kind of advanced query grouping is a very important factor. Grouping helps in executing special function like sum, max average etc to be performed on certain groups of data inside the date result set. Grouping is done with the help of the Group method. Below is an example of the basic group functionality.     int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };         var numberGroups =         from num in numbers         group num by num % 5 into numGroup         select new { Remainder = numGroup.Key, Numbers = numGroup };  In the above example we have grouped the values based on the reminder left over when divided by 5. First we are grouping the values based on the reminder when divided by 5 into the numgroup variable.  numGroup.Key gives the value of the key on which the grouping has been applied. And the numGroup itself contains all the records that are contained in that group. Below is another example to explain the same. string[] words = { "blueberry", "abacus", "banana", "apple", "cheese" };         var wordGroups =         from num in words         group num by num[0] into grp         select new { FirstLetter = grp.Key, Words = grp }; In the above example we are grouping the value with the first character of the string (num[0]). Just like the order operator the group by clause also allows us to write our own logic for the Equal comparison (That means we can group Item by ignoring case also by writing out own implementation). For this we need to pass an object that implements the IEqualityComparer<string> interface. Below is an example. public class AnagramEqualityComparer : IEqualityComparer<string> {     public bool Equals(string x, string y) {         return getCanonicalString(x) == getCanonicalString(y);     }      public int GetHashCode(string obj) {         return getCanonicalString(obj).GetHashCode();     }         private string getCanonicalString(string word) {         char[] wordChars = word.ToCharArray();         Array.Sort<char>(wordChars);         return new string(wordChars);     } }  string[] anagrams = {"from   ", " salt", " earn", "  last   ", " near "}; var orderGroups = anagrams.GroupBy(w => w.Trim(), new AnagramEqualityComparer()); Vikram  

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  • Enhancing performance in Entity Framework applications by precompiling LINQ to Entities queries

    - by nikolaosk
    This is going to be the tenth post of a series of posts regarding ASP.Net and the Entity Framework and how we can use Entity Framework to access our datastore. You can find the first one here , the second one here , the third one here , the fourth one here , the fifth one here ,the sixth one here ,the seventh one here ,the eighth one here and the ninth one here . I have a post regarding ASP.Net and EntityDataSource . You can read it here .I have 3 more posts on Profiling Entity Framework applications...(read more)

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  • LINQ: Enhancing Distinct With The SelectorEqualityComparer

    - by Paulo Morgado
    On my last post, I introduced the PredicateEqualityComparer and a Distinct extension method that receives a predicate to internally create a PredicateEqualityComparer to filter elements. Using the predicate, greatly improves readability, conciseness and expressiveness of the queries, but it can be even better. Most of the times, we don’t want to provide a comparison method but just to extract the comaprison key for the elements. So, I developed a SelectorEqualityComparer that takes a method that extracts the key value for each element. Something like this: public class SelectorEqualityComparer<TSource, Tkey> : EqualityComparer<TSource> where Tkey : IEquatable<Tkey> { private Func<TSource, Tkey> selector; public SelectorEqualityComparer(Func<TSource, Tkey> selector) : base() { this.selector = selector; } public override bool Equals(TSource x, TSource y) { Tkey xKey = this.GetKey(x); Tkey yKey = this.GetKey(y); if (xKey != null) { return ((yKey != null) && xKey.Equals(yKey)); } return (yKey == null); } public override int GetHashCode(TSource obj) { Tkey key = this.GetKey(obj); return (key == null) ? 0 : key.GetHashCode(); } public override bool Equals(object obj) { SelectorEqualityComparer<TSource, Tkey> comparer = obj as SelectorEqualityComparer<TSource, Tkey>; return (comparer != null); } public override int GetHashCode() { return base.GetType().Name.GetHashCode(); } private Tkey GetKey(TSource obj) { return (obj == null) ? (Tkey)(object)null : this.selector(obj); } } Now I can write code like this: .Distinct(new SelectorEqualityComparer<Source, Key>(x => x.Field)) And, for improved readability, conciseness and expressiveness and support for anonymous types the corresponding Distinct extension method: public static IEnumerable<TSource> Distinct<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> selector) where TKey : IEquatable<TKey> { return source.Distinct(new SelectorEqualityComparer<TSource, TKey>(selector)); } And the query is now written like this: .Distinct(x => x.Field) For most usages, it’s simpler than using a predicate.

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  • how to update child records when updating the Master table using Linq [closed]

    - by user20358
    I currently use a general repositry class that can update only a single table like so public abstract class MyRepository<T> : IRepository<T> where T : class { protected IObjectSet<T> _objectSet; protected ObjectContext _context; public MyRepository(ObjectContext Context) { _objectSet = Context.CreateObjectSet<T>(); _context = Context; } public IQueryable<T> GetAll() { return _objectSet.AsQueryable(); } public IQueryable<T> Find(Expression<Func<T, bool>> filter) { return _objectSet.Where(filter); } public void Add(T entity) { _objectSet.AddObject(entity); _context.ObjectStateManager.ChangeObjectState(entity, System.Data.EntityState.Added); _context.SaveChanges(); } public void Update(T entity) { _context.ObjectStateManager.ChangeObjectState(entity, System.Data.EntityState.Modified); _context.SaveChanges(); } public void Delete(T entity) { _objectSet.Attach(entity); _context.ObjectStateManager.ChangeObjectState(entity, System.Data.EntityState.Deleted); _objectSet.DeleteObject(entity); _context.SaveChanges(); } } For every table class generated by my EDMX designer I create another class like this public class CustomerRepo : MyRepository<Customer> { public CustomerRepo (ObjectContext context) : base(context) { } } for any updates that I need to make to a particular table I do this: Customer CustomerObj = new Customer(); CustomerObj.Prop1 = ... CustomerObj.Prop2 = ... CustomerObj.Prop3 = ... CustomerRepo.Update(CustomerObj); This works perfectly well when I am updating just to the specific table called Customer. Now if I need to also update each row of another table which is a child of Customer called Orders what changes do I need to make to the class MyRepository. Orders table will have multiple records for a Customer record and multiple fields too, say for example Field1, Field2, Field3. So my questions are: 1.) If I only need to update Field1 of the Orders table for some rows based on a condition and Field2 for some other rows based on a different condition then what changes I need to do? 2.) If there is no such condition and all child rows need to be updated with the same value for all rows then what changes do I need to do? Thanks for taking the time. Look forward to your inputs...

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  • LINQ – Skip() and Take() methods

    - by nmarun
    I had this issue recently where I have an array of integers and I’m doing some Skip(n) and then a Take(m) on the collection. Here’s an abstraction of the code: 1: int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 }; 2: var taken = numbers.Skip(3).Take(3); 3: foreach (var i in taken) 4: { 5: Console.WriteLine(i); 6: } The output is as expected: 3, 9, 8 – skip the first three and then take the next three items. But, what happens if I do something like: 1: var taken = numbers.Skip(7).Take(5); In English – skip the first seven and the take the next 5 items from an array that contains only 10 elements. Think it’ll throw the IndexOutOfRangeException exception? Nope. These extension methods are a little smarter than that. Even though the user has requested more elements than what exists in the collection, the Take method only returns the first three thereby making the output of the program as: 7, 2, 0. The scenario is handled similarly when you do: 1: var taken = numbers.Take(5).Skip(7); This one takes the first 5 elements from the numbers array and then skips 7 of them. This is what is looks like in the debug mode: Just wanted to share this behavior.

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  • Several Small, Specific, MySQL Query Cache Questions

    - by Robbie
    I've look all over the web and in the questions asked here about MySQL caching and most of them seem very non-specific about a couple of questions that I have about performance and MySQL query caching. Specifically I want answers to these questions, assume for all questions that I have the query cache enabled and it is of type 2, or "DEMAND": Is the query cache per table, per database, or per server? Meaning if I have the cache size set to X and have T tables and D databases will I be caching TX, DX, or X amount of data? If I have table T1 which I regularly use the SQL_CACHE hint on for SELECT queries and table T2 which I never do, when I query T2 with a SELECT query will it check through the cache first before performing the query? *Note: I don't want to use the SQL_NO_CACHE for all T2 queries.* Assume the same situation as in question 2. If I alter (INSERT, DELETE) table T2 will any processing be done on the cache? For answers to 2 and 3, is this processing time negligible if T2 is constantly being altered and is the target of a majority of my SELECT queries?

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  • Optimizing MySQL update query

    - by Jernej Jerin
    This is currently my MySQL UPDATE query, which is called from program written in Java: String query = "UPDATE maxday SET DatePressureREL = (SELECT Date FROM ws3600 WHERE PressureREL = (SELECT MAX" + "(PressureREL) FROM ws3600 WHERE Date >= '" + Date + "') AND Date >= '" + Date + "' ORDER BY Date DESC LIMIT 1), " + "PressureREL = (SELECT PressureREL FROM ws3600 WHERE PressureREL = (SELECT MAX(PressureREL) FROM ws3600 " + "WHERE Date >= '" + Date + "') AND Date >= '" + Date + "' ORDER BY Date DESC LIMIT 1), ..."; try { s.execute(query); } catch (SQLException e) { System.out.println("SQL error"); } catch(Exception e) { e.printStackTrace(); } Let me explain first, what does it do. I have two tables, first is ws3600, which holds columns (Date, PressureREL, TemperatureOUT, Dewpoint, ...). Then I have second table, called maxday, which holds columns like DatePressureREL, PressureREL, DateTemperatureOUT, TemperatureOUT,... Now as you can see from an example, I update each column, the question is, is there a faster way? I am asking this, because I am calling MAX twice, first to find the Date for that value and secondly to find the actual value. Now I know that I could write like that: SELECT Date, PressureREL FROM ws3600 WHERE PressureREL = (SELECT MAX(PressureREL) FROM ws3600 WHERE Date >= '" + Date + "') AND Date >= '" + Date + "' ORDER BY Date DESC LIMIT 1 That way I get the Date of the max and the max value at the same time and then update with those values the data in maxday table. But the problem of this solution is, that I have to execute many queries, which as I understand takes alot more time compared to executing one long mysql query because of overhead in sending each query to the server. If there is no better way, which solution beetwen this two should I choose. The first, which only takes one query but is very unoptimized or the second which is beter in terms of optimization, but needs alot more queries which probably means that the preformance gain is lost because of overhead in sending each query to the server?

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  • Syntax Error in MySql StoredProc

    - by karthik
    I am using the below stored proc in mysql to generate the insert statements. I am getting the following error : Script line: 4 You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '\’”\’,',’ifnull(‘,column_name,’,””)’,',\’”\’)')) INTO @S' at line 12 What would be the syntax problem in that ? DELIMITER $$ DROP PROCEDURE IF EXISTS `InsGen` $$ CREATE DEFINER=`root`@`localhost` PROCEDURE `InsGen`(in_db varchar(20),in_table varchar(20),in_file varchar(100)) BEGIN declare Whrs varchar(500); declare Sels varchar(500); declare Inserts varchar(2000); declare tablename varchar(20); set tablename=in_table; select tablename; # Comma separated column names – used for Select select group_concat(concat(‘concat(\’”\’,',’ifnull(‘,column_name,’,””)’,',\’”\’)')) INTO @Sels from information_schema.columns where table_schema=’test’ and table_name=tablename; # Comma separated column names – used for Group By select group_concat(‘`’,column_name,’`') INTO @Whrs from information_schema.columns where table_schema=’test’ and table_name=tablename; #Main Select Statement for fetching comma separated table values set @Inserts=concat(“select concat(‘insert into “, in_db,”.”,tablename,” values(‘,concat_ws(‘,’,”,@Sels,”),’);’) from “, in_db,”.”,tablename,” group by “,@Whrs, ” INTO OUTFILE ‘”, in_file ,”‘”); PREPARE Inserts FROM @Inserts; EXECUTE Inserts; END $$ DELIMITER ;

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  • convert MsSql StoredPorcedure to MySql

    - by karthik
    I need to covert the following SP of MsSql To MySql. I am new to MySql.. Help needed. CREATE PROC InsertGenerator (@tableName varchar(100)) as --Declare a cursor to retrieve column specific information --for the specified table DECLARE cursCol CURSOR FAST_FORWARD FOR SELECT column_name,data_type FROM information_schema.columns WHERE table_name = @tableName OPEN cursCol DECLARE @string nvarchar(3000) --for storing the first half --of INSERT statement DECLARE @stringData nvarchar(3000) --for storing the data --(VALUES) related statement DECLARE @dataType nvarchar(1000) --data types returned --for respective columns SET @string='INSERT '+@tableName+'(' SET @stringData='' DECLARE @colName nvarchar(50) FETCH NEXT FROM cursCol INTO @colName,@dataType IF @@fetch_status<0 begin print 'Table '+@tableName+' not found, processing skipped.' close curscol deallocate curscol return END WHILE @@FETCH_STATUS=0 BEGIN IF @dataType in ('varchar','char','nchar','nvarchar') BEGIN SET @stringData=@stringData+'''''''''+ isnull('+@colName+','''')+'''''',''+' END ELSE if @dataType in ('text','ntext') --if the datatype --is text or something else BEGIN SET @stringData=@stringData+'''''''''+ isnull(cast('+@colName+' as varchar(2000)),'''')+'''''',''+' END ELSE IF @dataType = 'money' --because money doesn't get converted --from varchar implicitly BEGIN SET @stringData=@stringData+'''convert(money,''''''+ isnull(cast('+@colName+' as varchar(200)),''0.0000'')+''''''),''+' END ELSE IF @dataType='datetime' BEGIN SET @stringData=@stringData+'''convert(datetime,''''''+ isnull(cast('+@colName+' as varchar(200)),''0'')+''''''),''+' END ELSE IF @dataType='image' BEGIN SET @stringData=@stringData+'''''''''+ isnull(cast(convert(varbinary,'+@colName+') as varchar(6)),''0'')+'''''',''+' END ELSE --presuming the data type is int,bit,numeric,decimal BEGIN SET @stringData=@stringData+'''''''''+ isnull(cast('+@colName+' as varchar(200)),''0'')+'''''',''+' END SET @string=@string+@colName+',' FETCH NEXT FROM cursCol INTO @colName,@dataType END

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  • MySQL Connector: parameters not being added

    - by LookitsPuck
    Hey all! Looking at my query log for MySQL, I see my parameters aren't being added. Here's my code: MySqlConnection conn = new MySqlConnection(ApplicationVariables.ConnectionString()); MySqlCommand com = new MySqlCommand(); try { conn.Open(); com.Connection = conn; com.CommandText = String.Format(@"SELECT COUNT(*) AS totalViews FROM pr_postreleaseviewslog AS prvl WHERE prvl.dateCreated BETWEEN (@startDate) AND (@endDate) AND prvl.postreleaseID IN ({0})" , ids); com.CommandType = CommandType.Text; com.Parameters.Add(new MySqlParameter("@startDate", thisCampaign.Startdate)); com.Parameters.Add(new MySqlParameter("@endDate", endDate)); numViews = Convert.ToInt32(com.ExecuteScalar()); } catch (Exception ex) { } finally { conn.Dispose(); com.Dispose(); } Looking at the query log, I see this: SELECT COUNT(*) AS totalViews FROM pr_postreleaseviewslog AS prvl WHERE prvl.dateCreated BETWEEN (@startDate) AND (@endDate) AND prvl.postreleaseID IN (1,2) I've used the MySQL .NET connector on countless projects (I actually have a base class that takes care of opening these connections, and closing them with transactions, etc.). However, I took over this application, and here I am now. Thanks for the help!

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  • MySQL DDL error creating tables

    - by Alexandstein
    I am attempting to create tables for a MySQL database, but I am having some syntactical issues. It would seem that syntax checking is behaving differently between tables for some reason. While I've gotten all the other tables to go through, the table, 'stock' doesn't seem to be working, despite seeming to use the same syntax patterns. CREATE TABLE users ( user_id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT, username VARCHAR(30) NOT NULL, password CHAR(41) NOT NULL, date_joined DATETIME NOT NULL, funds DOUBLE UNSIGNED NOT NULL, PRIMARY KEY(user_id), UNIQUE KEY(username) ); CREATE TABLE owned_stocks ( id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT, user_id SMALLINT UNSIGNED NOT NULL, paid_price DOUBLE UNSIGNED NOT NULL, quantity MEDIUMINT UNSIGNED NOT NULL, purchase_date DATETIME NOT NULL, PRIMARY KEY(id) ); CREATE TABLE tracking_stocks ( ticker VARCHAR(5) NOT NULL, user_id SMALLINT UNSIGNED NOT NULL, PRIMARY KEY(ticker) ); CREATE TABLE stocks ( ticker VARCHAR(5) NOT NULL, last DOUBLE UNSIGNED NOT NULL, high DOUBLE UNSIGNED NOT NULL, low DOUBLE UNSIGNED NOT NULL, company_name VARCHAR(30) NOT NULL, last_updated INT UNSIGNED NOT NULL, change DOUBLE NOT NULL, percent_change DOUBLE NOT NULL, PRIMARY KEY(ticker) ); Am I just missing a really obvious syntactical issue? ERROR: #1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'change DOUBLE NOT NULL, percent_change DOUBLE NOT NULL, last DOUBLE' at line 4

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  • How to get MySQL database to appear on index.php

    - by Teddy Truong
    Hi, I have a submission form on my website (index.php) and I have the data(user submissions) being stored into a MySQL database. Right now, I have the user submitting a post and then the page directs them to an update.php which shows what they inputed. However, I want all of the data in the database in MySQL to be shown on the index.php. It's a lot like a comment system. User submits a post... and sees their post above the other submitted posts all on the same page. I think I'm missing AJAX... ? Here is the code for index.php <div align="center"> <p>&nbsp;</p> <h2 align="center" class="Title"><em><strong>REDACTED</strong></em></h2> <form id="form1" name="form1" method="post" action="update.php"> <hr /> <label><br /> <form action="update.php" method="post"> REDACTED: <input type="text" name="text" /> <input type="submit" /> </form> </label> </form> </div> On update.php I have this: ?php $text = $_POST['text']; $myString = "REDACTED"; mysql_connect ("db----.net", "-----3", "------------") or die ('Error: ' . mysql_error()); mysql_select_db ("-----------"); $query="INSERT INTO TextArea (ID, text) VALUES ('NULL', '".$text."')"; mysql_query($query) or die ('Error updating database'); echo " $myString "," $text "; ?> Thanks a lot!

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  • MySQL "OR MATCH" hangs (very slow) on multiple tables

    - by Kerry
    After learning how to do MySQL Full-Text search, the recommended solution for multiple tables was OR MATCH and then do the other database call. You can see that in my query below. When I do this, it just gets stuck in a "busy" state, and I can't access the MySQL database. SELECT a.`product_id`, a.`name`, a.`slug`, a.`description`, b.`list_price`, b.`price`, c.`image`, c.`swatch`, e.`name` AS industry, MATCH( a.`name`, a.`sku`, a.`description` ) AGAINST ( '%s' IN BOOLEAN MODE ) AS relevance FROM `products` AS a LEFT JOIN `website_products` AS b ON (a.`product_id` = b.`product_id`) LEFT JOIN ( SELECT `product_id`, `image`, `swatch` FROM `product_images` WHERE `sequence` = 0) AS c ON (a.`product_id` = c.`product_id`) LEFT JOIN `brands` AS d ON (a.`brand_id` = d.`brand_id`) INNER JOIN `industries` AS e ON (a.`industry_id` = e.`industry_id`) WHERE b.`website_id` = %d AND b.`status` = %d AND b.`active` = %d AND MATCH( a.`name`, a.`sku`, a.`description` ) AGAINST ( '%s' IN BOOLEAN MODE ) OR MATCH ( d.`name` ) AGAINST ( '%s' IN BOOLEAN MODE ) GROUP BY a.`product_id` ORDER BY relevance DESC LIMIT 0, 9 Any help would be greatly appreciated. EDIT All the tables involved are MyISAM, utf8_general_ci. Here's the EXPLAIN SELECT statement: id select_type table type possible_keys key key_len ref rows Extra 1 PRIMARY a ALL NULL NULL NULL NULL 16076 Using temporary; Using filesort 1 PRIMARY b ref product_id product_id 4 database.a.product_id 2 1 PRIMARY e eq_ref PRIMARY PRIMARY 4 database.a.industry_id 1 1 PRIMARY <derived2> ALL NULL NULL NULL NULL 23261 1 PRIMARY d eq_ref PRIMARY PRIMARY 4 database.a.brand_id 1 Using where 2 DERIVED product_images ALL NULL NULL NULL NULL 25933 Using where I don't know how to make that look neater -- sorry about that UPDATE it returns the query after 196 seconds (I think correctly). The query without multiple tables takes about .56 seconds (which I know is really slow, we plan on changing to solr or sphinx soon), but 196 seconds?? If we could add a number to the relevance if it was in the brand name ( d.name ), that would also work

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