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CREATE TRIGGER complex_constraint BEFORE INSERT ON my_table
FOR EACH ROW
BEGIN
    IF /*complex constraint*/(NEW.field1 IS NOT NULL AND NEW.field2 IS NOT NULL) OR  (NEW.field1 IS NULL AND NEW.field2 IS NULL) THEN
    SIGNAL SQLSTATE '45000' 
        SET MESSAGE_TEXT = 'complex constraint exception message';
    END IF;
END