Syntax:
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
http://dev.mysql.com/doc/refman/5.0/en/innodb-transaction-isolation.htm
l. InnoDB supports each of these levels in MySQL 5.0. The default level
is REPEATABLE READ.

To set the initial default global isolation level for mysqld, use the
--transaction-isolation option. See
http://dev.mysql.com/doc/refman/5.0/en/server-options.html.

URL: http://dev.mysql.com/doc/refman/5.0/en/set-transaction.html

