Re: LEFT JOIN and WHERE: identical or not and what is better?, etc.
2011/04/25 17:42 +0300, Andre Polykanine Here is the first one. We have two queries: SELECT `blogs`.* FROM `Blogs` LEFT JOIN `Users` ON `Blogs`.`UserId`=`Users`.`Id`; and the following one: SELECT `Blogs`.* FROM `Blogs`, `Users` WHERE `Blogs`.`UserId`=`Users`.`Id`; 1. Are they identical? 2. Which is better (faster, more optimal, more kosher, I mean, better style...)? 2011/04/25 10:16 -0500, Johnny Withers The only difference once MySQL parses these two queries is the first one is a LEFT JOIN, which will produce all records from the blogs table even if there is no matching record in the users table. The second query produces an INNER JOIN which means only rows with matching records in both tables will be returned. ... I prefer to write the INNER JOIN out though because it leaves my WHERE clause to do filtering. and it is usual to write all about the joining in the FROM-clause --the tables and the criterion for joining them-- and reserve the WHERE-clause for filtering the result: SELECT `blogs`.* FROM `Blogs` LEFT JOIN `Users` ON `Blogs`.`UserId` = `Users`.`Id` SELECT `Blogs`.* FROM `Blogs` JOIN `Users` ON `Blogs`.`UserId` = `Users`.`Id` That is, if you already had a table with the joined outcome, you would use the WHERE-clause to determine what of it enters into further processing. -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe:http://lists.mysql.com/mysql?unsub=arch...@jab.org
Re: LEFT JOIN and WHERE: identical or not and what is better?, etc.
I would go with join rather than where condition. 2011/4/26 Halász Sándor h...@tbbs.net 2011/04/25 17:42 +0300, Andre Polykanine Here is the first one. We have two queries: SELECT `blogs`.* FROM `Blogs` LEFT JOIN `Users` ON `Blogs`.`UserId`=`Users`.`Id`; and the following one: SELECT `Blogs`.* FROM `Blogs`, `Users` WHERE `Blogs`.`UserId`=`Users`.`Id`; 1. Are they identical? 2. Which is better (faster, more optimal, more kosher, I mean, better style...)? 2011/04/25 10:16 -0500, Johnny Withers The only difference once MySQL parses these two queries is the first one is a LEFT JOIN, which will produce all records from the blogs table even if there is no matching record in the users table. The second query produces an INNER JOIN which means only rows with matching records in both tables will be returned. ... I prefer to write the INNER JOIN out though because it leaves my WHERE clause to do filtering. and it is usual to write all about the joining in the FROM-clause --the tables and the criterion for joining them-- and reserve the WHERE-clause for filtering the result: SELECT `blogs`.* FROM `Blogs` LEFT JOIN `Users` ON `Blogs`.`UserId` = `Users`.`Id` SELECT `Blogs`.* FROM `Blogs` JOIN `Users` ON `Blogs`.`UserId` = `Users`.`Id` That is, if you already had a table with the joined outcome, you would use the WHERE-clause to determine what of it enters into further processing. -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe: http://lists.mysql.com/mysql?unsub=sureshkumar...@gmail.com -- Thanks Suresh Kuna MySQL DBA
Re: LEFT JOIN and WHERE: identical or not and what is better?, etc.
Hello Halбsz, Aha. So, I should write SELECT `Blogs`.* INNER JOIN `Users` ON `Users`.`Id`=`Blogs`.`UserId` instead of my original WHERE clause? Thanks! -- With best regards from Ukraine, Andre Skype: Francophile My blog: http://oire.org/menelion (mostly in Russian) Twitter: http://twitter.com/m_elensule Facebook: http://facebook.com/menelion Original message From: Halбsz Sбndor h...@tbbs.net To: Andre Polykanine Date created: , 7:00:03 AM Subject: LEFT JOIN and WHERE: identical or not and what is better?, etc. 2011/04/25 17:42 +0300, Andre Polykanine Here is the first one. We have two queries: SELECT `blogs`.* FROM `Blogs` LEFT JOIN `Users` ON `Blogs`.`UserId`=`Users`.`Id`; and the following one: SELECT `Blogs`.* FROM `Blogs`, `Users` WHERE `Blogs`.`UserId`=`Users`.`Id`; 1. Are they identical? 2. Which is better (faster, more optimal, more kosher, I mean, better style...)? 2011/04/25 10:16 -0500, Johnny Withers The only difference once MySQL parses these two queries is the first one is a LEFT JOIN, which will produce all records from the blogs table even if there is no matching record in the users table. The second query produces an INNER JOIN which means only rows with matching records in both tables will be returned. ... I prefer to write the INNER JOIN out though because it leaves my WHERE clause to do filtering. and it is usual to write all about the joining in the FROM-clause --the tables and the criterion for joining them-- and reserve the WHERE-clause for filtering the result: SELECT `blogs`.* FROM `Blogs` LEFT JOIN `Users` ON `Blogs`.`UserId` = `Users`.`Id` SELECT `Blogs`.* FROM `Blogs` JOIN `Users` ON `Blogs`.`UserId` = `Users`.`Id` That is, if you already had a table with the joined outcome, you would use the WHERE-clause to determine what of it enters into further processing. -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe:http://lists.mysql.com/mysql?unsub=arch...@jab.org
Re: LEFT JOIN and WHERE: identical or not and what is better?, etc.
2011/04/26 17:55 +0300, Andre Polykanine Aha. So, I should write SELECT `Blogs`.* INNER JOIN `Users` ON `Users`.`Id`=`Blogs`.`UserId` instead of my original WHERE clause? Thanks! I think so. -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe:http://lists.mysql.com/mysql?unsub=arch...@jab.org
