SET [GLOBAL | SESSION] TRANSACTION ISOLATION LEVEL { READ UNCOMMITTED | READ COMMITTED | REPEATABLE READ | SERIALIZABLE }
Sets the transaction isolation level for the global, whole session or the next transaction.
The default behavior is to set the isolation level for the next (not started) transaction. If you use the GLOBAL keyword, the statement sets the default transaction level globally for all new connections created from that point on (but not existing connections). You need the SUPER privilege to do this. Using the SESSION keyword sets the default transaction level for all future transactions performed on the current connection.
For description of each InnoDB transaction isolation level, see InnoDB transaction isolation. InnoDB supports each of these levels from MySQL 4.0.5 on. The default level is REPEATABLE READ.
You can set the default global isolation level for mysqld with --transaction-isolation=.... See Server options.