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.

An IF ... END IF block — like all other flow-control blocks used within stored routines — 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 ;

In this example, the inner IF is evaluated only if n is not equal to m.