MySQL:IF no procedimento armazenado

CREATE PROCEDURE someFunction ( a VARCHAR(256),  b VARCHAR(256) )
    BEGIN
        DECLARE haveAllVariables INT;
        SET haveAllVariables = 1;

    IF     a = "" THEN SET haveAllVariables = 0;
    ELSEIF b = "" THEN SET haveAllVariables = 0;
    END IF;