Even if you think you know everything about LEFT JOIN, I bet you will learn something or two in this post! The difference between the ON clause and the WHERE clause. A simple way to better understand a complex Matching-Condition with WHERE … IS NULL clause. The difference between the Matching-Conditions and the Where-conditions.
Archive for the ‘Cool MySQL Queries’ Category
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 [...]
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 [...]
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.