SQL 错误 (1093) 您不能在 FROM 子句中指定要更新的目标表和子查询

Question

有一个存储过程很好用！

DELIMITER //
CREATE PROCEDURE temp
(
IN `valcat` INT,
IN `valsubcat` INT
)

BEGIN

DECLARE tcount INT;

SELECT COUNT(*) INTO tcount
FROM subcat sz
WHERE sz.cat_id = valcat;
CASE
    WHEN tcount > 1 THEN
        DELETE sy
        FROM subcat sy
        WHERE sy.sub_id = valsubcat;
    ELSE
        DELETE sx, c
        FROM subcat sx, cat_name c
        WHERE sx.sub_id = valsubcat AND c.cat_id = valcat;
END CASE;

END //

DELIMITER ;

CALL temp(7, 14);

现在我想去掉 'valcat' 参数并用另一个表的子查询替换它。我以这种方式重写了我的存储过程：

DELIMITER //
CREATE PROCEDURE temp
(
IN `valsubcat` INT
)

BEGIN

DECLARE tcount INT;

SELECT COUNT(*) INTO tcount
FROM subcat sz
WHERE sz.cat_id = (SELECT sw.cat_id
                    FROM subcat sw
                    WHERE sw.sub_id = valsubcat);
CASE
    WHEN tcount > 1 THEN
        DELETE sy
        FROM subcat sy
        WHERE sy.sub_id = valsubcat;
    ELSE
        DELETE sx, c
        FROM subcat sx, cat_name c
        WHERE sx.sub_id = valsubcat AND c.cat_id = (SELECT sv.cat_id
                                                    FROM subcat sv
                                                    WHERE sv.sub_id = valsubcat);
END CASE;

END //

DELIMITER ;

CALL temp(14);

...但现在我得到了消息：

''SQL 错误 (1093) in statement #3: You can't specify target table 'sx' 用于更新 FORM 子句''

附：我还有一个问题：使用过程的第二种变体时性能会下降吗？

Answer 1

DELETE sx, c
FROM subcat sx, cat_name c
WHERE sx.sub_id = valsubcat AND c.cat_id = (SELECT sv.cat_id
                                            FROM subcat sv
                                            WHERE sv.sub_id = valsubcat);

是错误的，必须用类似的东西替换

DELETE sx, c
FROM subcat sx
JOIN cat_name c ON sx.sub_id = valsubcat
JOIN subcat sv ON c.cat_id = sv.cat_id AND sv.sub_id = valsubcat;