http://www.mysqldiary.com Exploring, Sharing and Discussing MySQL practice Mon, 18 Jul 2011 20:03:03 +0000 en hourly 1 http://wordpress.org/?v=3.3.1 http://www.mysqldiary.com/mysql-left-join/ http://www.mysqldiary.com/mysql-left-join/#comments Wed, 06 Jul 2011 09:23:47 +0000 Hazan Ilan http://www.mysqldiary.com/?p=234 Even if you think you know everything about LEFT JOIN, I bet you will learn something or two in this post! A reminder about “A LEFT JOIN B ON conditional_expr” The ON condition (in the expression “A LEFT JOIN B ON conditional_expr”) is used to decide how to retrieve rows from table B (Matching-Stage). If [...]

]]> http://www.mysqldiary.com/mysql-left-join/feed/ 8 http://www.mysqldiary.com/as-restoring-a-dump-table-into-the-mysql-master-you-better-get-some-sleep/ http://www.mysqldiary.com/as-restoring-a-dump-table-into-the-mysql-master-you-better-get-some-sleep/#comments Mon, 04 Apr 2011 08:09:45 +0000 Hazan Ilan http://www.mysqldiary.com/?p=198 Restoring a dump table into the MySQL master server can lead to serious replication delay. The massive inserts commands cause the Master and slaves to use most of their resources for replication. As a result, replication lag may increase dramatically (linear to the table size). To overcome the replication delay, caused by restoring the dump [...]

]]> http://www.mysqldiary.com/as-restoring-a-dump-table-into-the-mysql-master-you-better-get-some-sleep/feed/ 4 http://www.mysqldiary.com/if-you-copy-a-myisam-table-with-primary-key-don%e2%80%99t-forget-to-order-the-rows-first/ http://www.mysqldiary.com/if-you-copy-a-myisam-table-with-primary-key-don%e2%80%99t-forget-to-order-the-rows-first/#comments Mon, 14 Mar 2011 19:02:23 +0000 Hazan Ilan http://www.mysqldiary.com/?p=189 In a case that you copy (INSERT INTO .. SELECT ..) a MyISAM table that have a PRIMARY key, it is much faster to insert the new rows in the primary key order: The insertion will be faster: because the primary key cannot disabled, every insertion will also updates the primary key. In a case [...]

]]> http://www.mysqldiary.com/if-you-copy-a-myisam-table-with-primary-key-don%e2%80%99t-forget-to-order-the-rows-first/feed/ 2 http://www.mysqldiary.com/a-many-to-many-relationship-table-solving-the-exclude-relation-problem/ http://www.mysqldiary.com/a-many-to-many-relationship-table-solving-the-exclude-relation-problem/#comments Tue, 18 Jan 2011 12:36:07 +0000 Hazan Ilan http://www.mysqldiary.com/?p=138 I will start by defining the MySQL Many to Many relationship (Experts can skip to the next paragraph) What is A MySQL many to many relationship A MySQL many to many relationship is a relationship that is multi-valued in both directions. This type of relationship is helped by the use of a linking table. For [...]

]]> http://www.mysqldiary.com/a-many-to-many-relationship-table-solving-the-exclude-relation-problem/feed/ 9 http://www.mysqldiary.com/the-battle-between-force-index-and-the-query-optimizer/ http://www.mysqldiary.com/the-battle-between-force-index-and-the-query-optimizer/#comments Mon, 25 Oct 2010 21:15:41 +0000 Hazan Ilan http://www.mysqldiary.com/?p=146 Query Optimizer is a part of the server that takes a parsed SQL query and produces a query execution plan. MySQL Query Optimizer uses (as one of its parameters) the stored key distribution (Cardinality) mechanism to determine the order in which tables should be joined, when you perform a join on something other than a [...]

]]> http://www.mysqldiary.com/the-battle-between-force-index-and-the-query-optimizer/feed/ 2 http://www.mysqldiary.com/optimizing-the-mysql-in-comparison-operations-which-include-the-indexed-field/ http://www.mysqldiary.com/optimizing-the-mysql-in-comparison-operations-which-include-the-indexed-field/#comments Thu, 07 Oct 2010 12:06:13 +0000 Hazan Ilan http://www.mysqldiary.com/?p=128 The MySQL IN() Comparison Operator is said to be very quick if all the values are constants (the values are then evaluated and sorted first, and the search is done using a binary search). However, what if the field which the IN clause refers to, is part of the index used to execute the query? [...]

]]> http://www.mysqldiary.com/optimizing-the-mysql-in-comparison-operations-which-include-the-indexed-field/feed/ 5 http://www.mysqldiary.com/limited-select-count/ http://www.mysqldiary.com/limited-select-count/#comments Thu, 13 May 2010 21:52:43 +0000 Hazan Ilan http://www.mysqldiary.com/?p=98 A SELECT statement with COUNT returns the number of rows retrieved by the SELECT statement (see mysql select count). For performance reasons, the desired result is to limit that count. Including a LIMIT clause in the SELECT statement will not work since it only restricts the number of rows returned, which is always one. The [...]

]]> http://www.mysqldiary.com/limited-select-count/feed/ 23 http://www.mysqldiary.com/conditional-joins-in-mysql/ http://www.mysqldiary.com/conditional-joins-in-mysql/#comments Sun, 07 Feb 2010 19:43:12 +0000 Hazan Ilan http://www.mysqldiary.com/?p=71 One way to do a “Conditional Join” in MySQL is by using a “LEFT JOIN”.  Create a “LEFT JOIN” for each condition and combine the results into one column using an “IF” statement by the “SELECT” expression.  Here’s an example: Suppose you have three tables: questions: a table consisting of question ids, timestamps, and whether [...]

]]> http://www.mysqldiary.com/conditional-joins-in-mysql/feed/ 9 http://www.mysqldiary.com/selecting-records-from-one-table-that-dont-have-specific-relationship-in-a-second-table/ http://www.mysqldiary.com/selecting-records-from-one-table-that-dont-have-specific-relationship-in-a-second-table/#comments Sun, 17 Jan 2010 19:57:07 +0000 Hazan Ilan http://www.mysqldiary.com/?p=58 Question: How can you select records from one table that doesn’t have a specific ID in a second, many-to-many table?   In other words, imagine you have a questions table (A), categories table (B), and a many-to-many relationship link table (L).  This setup allows a question to have several categories.  How can you find questions that [...]

]]> http://www.mysqldiary.com/selecting-records-from-one-table-that-dont-have-specific-relationship-in-a-second-table/feed/ 6 http://www.mysqldiary.com/how-to-produce-random-rows-from-a-table/ http://www.mysqldiary.com/how-to-produce-random-rows-from-a-table/#comments Sun, 29 Nov 2009 04:10:30 +0000 Hazan Ilan http://www.mysqldiary.com/?p=20 A while ago, I were searching for a way to produce random rows from a table in MySQL. I found several solutions but none of them satisfied me. Of course, I could use  a combined logic of MySQL and a programming language. For example by producing random numbers in PHP and using them in the MySQL query in a IN clouse.  However, I [...]

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