m CREATE TRIGGER check_room_availability_trigger BEFORE INSERT ON has_BookingRel FOR EACH ROW BEGIN DECLARE available_rooms INT; SELECT room_num - IFNULL(SUM(booked_quantity), 0) INTO available_rooms FROM room_types LEFT JOIN has_BookingRel ON room_types.id = has_BookingRel.room_type_id WHERE room_types.id = NEW.room_type_id; GROUP BY room_types.id, room_types.name, room_types.price; IF available_rooms < NEW.booked_quantity THEN SIGNAL SQLSTATE “45000” SET MESSAGE_TEXT = ‘房间数量不足’; END IF; END;1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'GROUP BY room_types.id, room_types.name, room_types.price; IF available_rooms ’ at line 12

在该触发器中，语法有误。出现错误的位置是在查询语句后的 GROUP BY 子句，因为在条件判断语句之前不能有 GROUP BY 子句。

以下是修改后的代码：

CREATE TRIGGER check_room_availability_trigger 
BEFORE INSERT ON has_BookingRel 
FOR EACH ROW 
BEGIN 
    DECLARE available_rooms INT; 
    SELECT room_num - IFNULL(SUM(booked_quantity), 0) INTO available_rooms 
    FROM room_types 
    LEFT JOIN has_BookingRel ON room_types.id = has_BookingRel.room_type_id 
    WHERE room_types.id = NEW.room_type_id;
    
    IF available_rooms < NEW.booked_quantity THEN 
        SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = '房间数量不足'; 
    END IF; 
END;

修改后的代码已经删除了 GROUP BY 子句，应该可以正常工作了。