SET [GLOBAL | SESSION] TRANSACTION ISOLATION LEVEL
  {
       READ UNCOMMITTED
     | READ COMMITTED
     | REPEATABLE READ
     | SERIALIZABLE
   }

This statement sets the transaction isolation level for the next transaction, globally, or for the current session.

The default behavior of SET TRANSACTION is to set the isolation level for the next (not yet 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. Existing connections are unaffected. 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 descriptions of each InnoDB transaction isolation level, see Section 13.2.11.3, “InnoDB and TRANSACTION ISOLATION LEVEL”. InnoDB supports each of these levels from MySQL 4.0.5 on. The default level is REPEATABLE READ.

To set the initial default global isolation level for mysqld, use the --transaction-isolation option. See Section 5.1.2, “Command Options”.