类似语法上的问题，其实可以参考一下手册中的说明和例子。

MySQL 5.4 Reference Manual12.8.6.1. IF Statement

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 Section 11.3, “Control Flow Functions”.

An IF ... END IF block, like all other flow-control blocks used within stored programs, must be terminated with a semicolon, as shown in this example:

DELIMITER //

CREATE FUNCTION SimpleCompare(n INT, m INT)

RETURNS VARCHAR(20)

BEGIN

DECLARE s VARCHAR(20);

IF n > m THEN SET s = '>';

ELSEIF n = m THEN SET s = '=';

ELSE SET s = '

END IF;

SET s = CONCAT(n, ' ', s, ' ', m);

RETURN s;

END //

DELIMITER ;

As with other flow-control constructs, IF ... END IF blocks may be nested within other flow-control constructs, including other IF statements. Each IF must be terminated by its own END IF followed by a semicolon. You can use indentation to make nested flow-control blocks more easily readable by humans (although this is not required by MySQL), as shown here:

DELIMITER //

CREATE FUNCTION VerboseCompare (n INT, m INT)

RETURNS VARCHAR(50)

BEGIN

DECLARE s VARCHAR(50);

IF n = m THEN SET s = 'equals';

ELSE

IF n > m THEN SET s = 'greater';

ELSE SET s = 'less';

END IF;

SET s = CONCAT('is ', s, ' than');

END IF;

SET s = CONCAT(n, ' ', s, ' ', m, '.');

RETURN s;

END //

DELIMITER ;