Syntax:
IF search_condition THEN statement_list
    [ELSEIF search_condition THEN statement_list] ...
    [ELSE statement_list]
END IF

IF implements a basic conditional construct. If the search_condition
evaluates to true, the corresponding SQL statement list is executed. If
no search_condition matches, the statement list in the ELSE clause is
executed. Each statement_list consists of one or more statements.

*Note*: There is also an IF() function, which differs from the IF
statement described here. See
http://dev.mysql.com/doc/refman/5.1/en/control-flow-functions.html.

URL: http://dev.mysql.com/doc/refman/5.1/en/if-statement.html

