SET TRANSACTION syntax

SET TRANSACTION ISOLATION LEVEL { READ UNCOMMITTED
    | READ COMMITTED
    | REPEATABLE READ
    | SERIALIZABLE }

SET TRANSACTION statement does nothing. It was implemented to maintain compatibility with 3rd party MySQL client libraries, connectors, and frameworks that may need to run this statement when connecting.

Example:

mysql> SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;
Query OK, 0 rows affected (0.00 sec)