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• Partition Wise Joins II

  - by jean-pierre.dijcks

  One of the things that I did not talk about in the initial partition wise join post was the effect it has on resource allocation on the database server. When Oracle applies a different join method - e.g. not PWJ - what you will see in SQL Monitor (in Enterprise Manager) or in an Explain Plan is a set of producers and a set of consumers. The producers scan the tables in the the join. If there are two tables the producers first scan one table, then the other. The producers thus provide data to the consumers, and when the consumers have the data from both scans they do the join and give the data to the query coordinator. Now that behavior means that if you choose a degree of parallelism of 4 to run such query with, Oracle will allocate 8 parallel processes. Of these 8 processes 4 are producers and 4 are consumers. The consumers only actually do work once the producers are fully done with scanning both sides of the join. In the plan above you can see that the producers access table SALES [line 11] and then do a PX SEND [line 9]. That is the producer set of processes working. The consumers receive that data [line 8] and twiddle their thumbs while the producers go on and scan CUSTOMERS. The producers send that data to the consumer indicated by PX SEND [line 5]. After receiving that data [line 4] the consumers do the actual join [line 3] and give the data to the QC [line 2]. BTW, the myth that you see twice the number of processes due to the setting PARALLEL_THREADS_PER_CPU=2 is obviously not true. The above is why you will see 2 times the processes of the DOP. In a PWJ plan the consumers are not present. Instead of producing rows and giving those to different processes, a PWJ only uses a single set of processes. Each process reads its piece of the join across the two tables and performs the join. The plan here is notably different from the initial plan. First of all the hash join is done right on top of both table scans [line 8]. This query is a little more complex than the previous so there is a bit of noise above that bit of info, but for this post, lets ignore that (sort stuff). The important piece here is that the PWJ plan typically will be faster and from a PX process number / resources typically cheaper. You may want to look out for those plans and try to get those to appear a lot... CREDITS: credits for the plans and some of the info on the plans go to Maria, as she actually produced these plans and is the expert on plans in general... You can see her talk about explaining the explain plan and other optimizer stuff over here: ODTUG in Washington DC, June 27 - July 1 On the Optimizer blog At OpenWorld in San Francisco, September 19 - 23 Happy joining and hope to see you all at ODTUG and OOW...

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• How to perform Cross Join with Linq

  - by berthin

  Cross join consists to perform a Cartesian product of two sets or sequences. The following example shows a simple Cartesian product of the sets A and B: A (a1, a2) B (b1, b2) => C (a1 b1,            a1 b2,            a2 b1,            a2, b2 ) is the Cartesian product's result. Linq to Sql allows using Cross join operations. Cross join is not equijoin, means that no predicate expression of equality in the Join clause of the query. To define a cross join query, you can use multiple from clauses. Note that there's no explicit operator for the cross join. In the following example, the query must join a sequence of Product with a sequence of Pricing Rules: 1: //Fill the data source 2: var products = new List<Product> 3: { 4: new Product{ProductID="P01",ProductName="Amaryl"}, 5: new Product {ProductID="P02", ProductName="acetaminophen"} 6: }; 7:  8: var pricingRules = new List<PricingRule> 9: { 10: new PricingRule {RuleID="R_1", RuleType="Free goods"}, 11: new PricingRule {RuleID="R_2", RuleType="Discount"}, 12: new PricingRule {RuleID="R_3", RuleType="Discount"} 13: }; 14: 15: //cross join query 16: var crossJoin = from p in products 17: from r in pricingRules 18: select new { ProductID = p.ProductID, RuleID = r.RuleID };   Below the definition of the two entities using in the above example.   1: public class Product 2: { 3: public string ProductID { get; set; } 4: public string ProductName { get; set; } 5: } 1: public class PricingRule 2: { 3: public string RuleID { get; set; } 4: public string RuleType { get; set; } 5: }   Doing this: 1: foreach (var result in crossJoin) 2: { 3: Console.WriteLine("({0} , {1})", result.ProductID, result.RuleID); 4: }   The output should be similar on this:   ( P01   -    R_1 )   ( P01   -    R_2 )   ( P01   -    R_3 )   ( P02   -    R_1 )   ( P02   -    R_2 )   ( P02   -    R_3) Conclusion Cross join operation is useful when performing a Cartesian product of two sequences object. However, it can produce very large result sets that may caused a problem of performance. So use with precautions :)

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• Linq, double left join and double count

  - by Fabian Vilers

  Hi! I'm looking to translate this SQL statement to a well working & performant LINQ command. I've managed to have the first count working using the grouping count and key members, but don't know how to get the second count. select main.title, count(details.id) as details, count(messages.id) as messages from main left outer join details on main.id = details.mainid left outer join messages on details.id = messages.detailid group by main.title Any advice is welcome! Fabian

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• JPA : optimize EJB-QL query involving large many-to-many join table

  - by Fabien

  Hi all. I'm using Hibernate Entity Manager 3.4.0.GA with Spring 2.5.6 and MySql 5.1. I have a use case where an entity called Artifact has a reflexive many-to-many relation with itself, and the join table is quite large (1 million lines). As a result, the HQL query performed by one of the methods in my DAO takes a long time. Any advice on how to optimize this and still use HQL ? Or do I have no choice but to switch to a native SQL query that would perform a join between the table ARTIFACT and the join table ARTIFACT_DEPENDENCIES ? Here is the problematic query performed in the DAO : @SuppressWarnings("unchecked") public List<Artifact> findDependentArtifacts(Artifact artifact) { Query query = em.createQuery("select a from Artifact a where :artifact in elements(a.dependencies)"); query.setParameter("artifact", artifact); List<Artifact> list = query.getResultList(); return list; } And the code for the Artifact entity : package com.acme.dependencytool.persistence.model; import java.util.ArrayList; import java.util.List; import javax.persistence.CascadeType; import javax.persistence.Column; import javax.persistence.Entity; import javax.persistence.FetchType; import javax.persistence.GeneratedValue; import javax.persistence.Id; import javax.persistence.JoinColumn; import javax.persistence.JoinTable; import javax.persistence.ManyToMany; import javax.persistence.Table; import javax.persistence.UniqueConstraint; @Entity @Table(name = "ARTIFACT", uniqueConstraints={@UniqueConstraint(columnNames={"GROUP_ID", "ARTIFACT_ID", "VERSION"})}) public class Artifact { @Id @GeneratedValue @Column(name = "ID") private Long id = null; @Column(name = "GROUP_ID", length = 255, nullable = false) private String groupId; @Column(name = "ARTIFACT_ID", length = 255, nullable = false) private String artifactId; @Column(name = "VERSION", length = 255, nullable = false) private String version; @ManyToMany(cascade=CascadeType.ALL, fetch=FetchType.EAGER) @JoinTable( name="ARTIFACT_DEPENDENCIES", joinColumns = @JoinColumn(name="ARTIFACT_ID", referencedColumnName="ID"), inverseJoinColumns = @JoinColumn(name="DEPENDENCY_ID", referencedColumnName="ID") ) private List<Artifact> dependencies = new ArrayList<Artifact>(); public Long getId() { return id; } public void setId(Long id) { this.id = id; } public String getGroupId() { return groupId; } public void setGroupId(String groupId) { this.groupId = groupId; } public String getArtifactId() { return artifactId; } public void setArtifactId(String artifactId) { this.artifactId = artifactId; } public String getVersion() { return version; } public void setVersion(String version) { this.version = version; } public List<Artifact> getDependencies() { return dependencies; } public void setDependencies(List<Artifact> dependencies) { this.dependencies = dependencies; } } Thanks in advance. EDIT 1 : The DDLs are generated automatically by Hibernate EntityMananger based on the JPA annotations in the Artifact entity. I have no explicit control on the automaticaly-generated join table, and the JPA annotations don't let me explicitly set an index on a column of a table that does not correspond to an actual Entity (in the JPA sense). So I guess the indexing of table ARTIFACT_DEPENDENCIES is left to the DB, MySQL in my case, which apparently uses a composite index based on both clumns but doesn't index the column that is most relevant in my query (DEPENDENCY_ID). mysql describe ARTIFACT_DEPENDENCIES; +---------------+------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +---------------+------------+------+-----+---------+-------+ | ARTIFACT_ID | bigint(20) | NO | MUL | NULL | | | DEPENDENCY_ID | bigint(20) | NO | MUL | NULL | | +---------------+------------+------+-----+---------+-------+ EDIT 2 : When turning on showSql in the Hibernate session, I see many occurences of the same type of SQL query, as below : select dependenci0_.ARTIFACT_ID as ARTIFACT1_1_, dependenci0_.DEPENDENCY_ID as DEPENDENCY2_1_, artifact1_.ID as ID1_0_, artifact1_.ARTIFACT_ID as ARTIFACT2_1_0_, artifact1_.GROUP_ID as GROUP3_1_0_, artifact1_.VERSION as VERSION1_0_ from ARTIFACT_DEPENDENCIES dependenci0_ left outer join ARTIFACT artifact1_ on dependenci0_.DEPENDENCY_ID=artifact1_.ID where dependenci0_.ARTIFACT_ID=? Here's what EXPLAIN in MySql says about this type of query : mysql explain select dependenci0_.ARTIFACT_ID as ARTIFACT1_1_, dependenci0_.DEPENDENCY_ID as DEPENDENCY2_1_, artifact1_.ID as ID1_0_, artifact1_.ARTIFACT_ID as ARTIFACT2_1_0_, artifact1_.GROUP_ID as GROUP3_1_0_, artifact1_.VERSION as VERSION1_0_ from ARTIFACT_DEPENDENCIES dependenci0_ left outer join ARTIFACT artifact1_ on dependenci0_.DEPENDENCY_ID=artifact1_.ID where dependenci0_.ARTIFACT_ID=1; +----+-------------+--------------+--------+-------------------+-------------------+---------+---------------------------------------------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------------+--------+-------------------+-------------------+---------+---------------------------------------------+------+-------+ | 1 | SIMPLE | dependenci0_ | ref | FKEA2DE763364D466 | FKEA2DE763364D466 | 8 | const | 159 | | | 1 | SIMPLE | artifact1_ | eq_ref | PRIMARY | PRIMARY | 8 | dependencytooldb.dependenci0_.DEPENDENCY_ID | 1 | | +----+-------------+--------------+--------+-------------------+-------------------+---------+---------------------------------------------+------+-------+ EDIT 3 : I tried setting the FetchType to LAZY in the JoinTable annotation, but I then get the following exception : Hibernate: select artifact0_.ID as ID1_, artifact0_.ARTIFACT_ID as ARTIFACT2_1_, artifact0_.GROUP_ID as GROUP3_1_, artifact0_.VERSION as VERSION1_ from ARTIFACT artifact0_ where artifact0_.GROUP_ID=? and artifact0_.ARTIFACT_ID=? 51545 [btpool0-2] ERROR org.hibernate.LazyInitializationException - failed to lazily initialize a collection of role: com.acme.dependencytool.persistence.model.Artifact.dependencies, no session or session was closed org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: com.acme.dependencytool.persistence.model.Artifact.dependencies, no session or session was closed at org.hibernate.collection.AbstractPersistentCollection.throwLazyInitializationException(AbstractPersistentCollection.java:380) at org.hibernate.collection.AbstractPersistentCollection.throwLazyInitializationExceptionIfNotConnected(AbstractPersistentCollection.java:372) at org.hibernate.collection.AbstractPersistentCollection.readSize(AbstractPersistentCollection.java:119) at org.hibernate.collection.PersistentBag.size(PersistentBag.java:248) at com.acme.dependencytool.server.DependencyToolServiceImpl.createArtifactViewBean(DependencyToolServiceImpl.java:93) at com.acme.dependencytool.server.DependencyToolServiceImpl.createArtifactViewBean(DependencyToolServiceImpl.java:109) at com.acme.dependencytool.server.DependencyToolServiceImpl.search(DependencyToolServiceImpl.java:48) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) at java.lang.reflect.Method.invoke(Method.java:597) at com.google.gwt.user.server.rpc.RPC.invokeAndEncodeResponse(RPC.java:527) at com.google.gwt.user.server.rpc.RemoteServiceServlet.processCall(RemoteServiceServlet.java:166) at com.google.gwt.user.server.rpc.RemoteServiceServlet.doPost(RemoteServiceServlet.java:86) at javax.servlet.http.HttpServlet.service(HttpServlet.java:637) at javax.servlet.http.HttpServlet.service(HttpServlet.java:717) at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:487) at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:362) at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216) at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:181) at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:729) at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:405) at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152) at org.mortbay.jetty.handler.RequestLogHandler.handle(RequestLogHandler.java:49) at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152) at org.mortbay.jetty.Server.handle(Server.java:324) at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:505) at org.mortbay.jetty.HttpConnection$RequestHandler.content(HttpConnection.java:843) at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:647) at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:205) at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:380) at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:395) at org.mortbay.thread.QueuedThreadPool$PoolThread.run(QueuedThreadPool.java:488)

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• INNER JOIN syntax for mySQL using phpmyadmin

  - by David van Dugteren

  SELECT Question.userid, user.uid FROM `question` WHERE NOT `userid`=2 LIMIT 0, 60 INNER JOIN `user` ON `question`.userid=`user`.uid ORDER BY `question`.userid returns Error: 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 'INNER JOIN User ON question.userid=user.uid ORDER BY question.userid' at line 5 Can't for the life of me figure out what I'm doing wrong here.

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• self join- how to use the aggregate functions

  - by Ranjana

  self join- how to use the aggregate functions select a.tablename, b.TableName,b.UserName from Employee a inner join Employee b on a.ColumnValue=b.ColumnValue and and a.TableName <> b.TableName and a.UserName=b.UserName and also to check whether the same user has count of records i.e Employee a = count of records of Employee b. how to add count function over here

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• mysql count, distinct, join? COnfusion

  - by calum

  hello i have 2 tables: tblItems ID | orderID | productID 1 1 2 2 1 2 3 2 1 4 3 2 tblProducts productID | productName 1 ABC 2 DEF im attempting to find the most popular Product based on whats in "tblItems", and display the product Name and the number of times it appears in the tblItems table. i can get mysql to count up the total like: $sql="SELECT COUNT(productID) AS CountProductID FROM tblItems"; but i can't figure out how to join the products table on..if i try LEFT JOIN the query goes horribly wrong hopefully thats not too confusing..thankss

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• Linq Left Outer Join

  - by Neil

  I am new to LINQ and am trying to convert a SQL query to LINQ: SQL: left outer join PRODUCT_BEST_USE pbu on pbu.PRODUCT_GUID = @uProductId and pbu.BEST_USE_GUID = bu.BEST_USE_GUID LINQ: from PBU in PRODUCT_BEST_USE.Where(PBU=>PBU.PRODUCT_GUID == p.PRODUCT_GUID).DefaultIfEmpty() When I add and PBU.BEST_USE_GUID equals BU.BEST_USE_GUID, I get an error: "A query body must end with a select clause or a group clause" Here is the full Linq query: from p in PRODUCT join BU in BEST_USE on p.CATEGORY_GUID equals BU.CATEGORY_GUID from PBU in PRODUCT_BEST_USE.Where(PBU=>PBU.PRODUCT_GUID == p.PRODUCT_GUID).DefaultIfEmpty() and PBU.BEST_USE_GUID equals BU.BEST_USE_GUID where p.PRODUCT_GUID == new Guid("d317752b-581d-4f43-92fa-4a4af91009f5") select new { BU.NAME, PBU.PRODUCT_BEST_USE_GUID }

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• What are the uses for Cross Join?

  - by vanslly

  A cross join performs a cartesian product on the tuples of the two sets. SELECT * FROM Table1 CROSS JOIN Table2 Which circumstances render such an SQL operation particularly useful?

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• SQL join different tables depending on row information

  - by Xster

  Hi, I think this question hasn't be asked yet. Suppose I have table A with a field that can be either 1 or 2 How do I select such that for each row in table A, if the field is 1, join the select with table B and if the field is 2, join the select with table C?

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• NHibernate Left Outer Join

  - by Matthew

  I'm looking to create a Left outer join Nhibernate query with multiple on statements akin to this: SELECT * FROM [Database].[dbo].[Posts] p LEFT JOIN [Database].[dbo].[PostInteractions] i ON p.PostId = i.PostID_TargetPost And i.UserID_ActingUser = 202 I've been fooling around with the critera and aliases, but I haven't had any luck figuring out how do to this. Any suggestions?

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• SQL - How to join on similar (not exact) columns

  - by BlueRaja

  I have two tables which get updated at almost the exact same time - I need to join on the datetime column. I've tried this: SELECT * FROM A, B WHERE ABS(DATEDIFF(second, A.Date_Time, B.Date_Time) = ( SELECT MIN(ABS(DATEDIFF(second, A.Date_Time, B2.Date_Time))) FROM B AS B2 ) But it tells me: Multiple columns are specified in an aggregated expression containing an outer reference. If an expression being aggregated contains an outer reference, then that outer reference must be the only column referenced in the expression. How can I join these tables?

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• SQL join produces one result only

  - by Rami

  Can anyone please tell me why this result is generation only one results? taking in mind that everything is set right and the three tables are populated correctly, i took out the group_concat and it worked but of course with a php undefined index error! SELECT `songs`.`song_name`, `songs`.`add_date`, `songs`.`song_id`, `songs`.`song_picture`, group_concat(DISTINCT artists.artist_name) as artist_name FROM (`songs`) JOIN `mtm_songs_artists` ON `songs`.`song_id` = `mtm_songs_artists`.`song_id` JOIN `artists` ON `artists`.`artist_id` = `mtm_songs_artists`.`artist_id` ORDER BY `songs`.`song_id` DESC LIMIT 10 so i'm guessing it's something related to group_concat. best regards, Rami

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• Using CASE Statements in LEFT OUTER JOIN in SQL

  - by s khan

  I've got a scenario where I want to switch on two different tables in an outer join. It goes something like this:- select mytable.id, yourtable.id from mytable left outer join (case when mytable.id = 2 then table2 yourtable on table1.id = table2.id else table3 yourtable on table1.id = table3.id end) ...but it doesn't work. Any suggestions?

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• web application , join event on facebook using php api

  - by Mohammed Al-shareif

  enter code herei created my events on facebook throw php api (form my website) ...every thing its fine with me and the events appear 100% on FB calendar , i got the FB event id and save to my db , enter code here my users can join the same event on my website i need them to join the same event on facebook also i have the facebook event id , and user accept my permission to access his/here calendar please anybody help!!!

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• join two table for insert data in database in android

  - by shadi

  I have two table(t1,t2) in android, t1 has a primary key that it is foreign key for t2,i want to insert data to this tables,is it necessary to join these two table?if yes, what is code for join these table? i insert data in one of them like this: public long insertQuote(String Quote,int Count1 ) { ContentValues initialValues = new ContentValues(); initialValues.put(GoodName, Quote); initialValues.put(CartID, Count1); return db.insert(DATABASE_TABLE, null, initialValues); }

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• linq to entites left outer join

  - by parminder

  I am struggling linq to entities left outer join. I have two entities (tables): Listings { ListingID, MakeID (nullable) } Makes { MakeID, Description } I want to write something like this in LINQ: select listings.listingID ,listings.makeid , IsNull(makes.Description, 'NA') from listings left outer join makes on listings.makeid = makes.makeid

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• Appending a prefix when using join in Perl

  - by syker

  I have an array of strings that I would like to use the join function on. However, I would like to prefix each string with the same string. Can I do this in one line as opposed to iterating through the array first and changing each value before using join?

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• Data retrieval and Join operations with cluster db server

  - by Goerge

  If any database spreads across multiple servers (ex. Microsoft Sql Server), how can we do join or filter operations. In my scenario, if suppose: A single table spreads across multiple servers how can we filter rows based on user input? If master table is there on one db server and transaction table is at another db server, how can we do join operations? Please let me know how can we achieve this and where can I get more details about this?

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• entity framework join

  - by Luca Romagnoli

  Hi, i have 2 table (user, user_profile) without a explicit relationship in the sql db. and i can't add it to the db. so, i can't do this: db.user.include("user_profile") the attribute in for the join is user_id is possible do anything like this? db.user.join("user_profile On user.id = user_profile.user_id") How can i do that? thanks

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• whats the best way to join an array into a readable list

  - by Ross

  I've got an array listing days of the week: days = ['Monday', 'Tuesday', 'Wednesday'] Whats the easiest / best way to output it in a human readable format: Monday, Tuesday and Wednesday The best I have is a rather ugly: ', '.join(days[:-2]+['']) + ' and '.join(days[-2:])

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• LINQ 2 SQL:Left outer join for my SQL statement

  - by Shyju

  Can any one tell me the LINQ 2 SQL version (in vb.net) of the below Left outer join query.I am trying to get all Employees with name "Shyju" and their address line 1 if it exist in the address table SELECT E.EMPLOYEE_NAME, E.AGE,A.ADRESS_LINE1 FROM EMPLOYEE_MASTER E LEFT OUTER JOIN ADDRESS_MASTER A ON E.ID=A.EMPLOYEE_ID WHERE E.EMPLOYEE_NAME='Shyju' Thanks in advance

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• Python: Split, strip, and join in one line

  - by PandemoniumSyndicate

  I'm curious if their is some python magic I may not know to accomplish a bit of frivolity given the line: csvData.append(','.join([line.split(":").strip() for x in L])) I'm attempting to split a line on :, trim whitespace around it, and join on , problem is, since the array is returned from line.split(":"), the for x in L #<== L doesn't exist! causes issues since I have no name for the array returned by line.split(":") So I'm curious if there is a sexy piece of syntax I could use to accomplish this in one shot? Cheers!

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• MySQL: Use CASE/ELSE value as join parameter

  - by DRJ

  I'm trying to join the NAME and PHOTO from USERS table to the TRANSACTIONS table based on who is the payer or payee. It keeps telling me can't find the table this -- What am I doing wrong? SELECT name,photo,amount,comment, ( CASE payer_id WHEN 72823 THEN payee_id ELSE payer_id END ) AS this FROM transactions RIGHT JOIN users ON (users.id=this) WHERE payee_id=72823 OR payer_id=72823

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• Join two results in Powershell

  - by Hinek

  I've got two CMDlets that return lists of objects. One returns objects of the type SPSolution, which contains the property Id, the other returns objects of the type SPFeature with a property SolutionId. Now I want to join/merge this data something like this: $f = Get-Feature $s = Get-Solution $result = <JOIN> $f $s <ON> $f.SolutionId = $s.Id <SELECT> FeatureName = $f.DisplayName, SolutionName = $s.Name

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