更新子查詢的WHERE子句中的UNION

Question

有什么原因為什么不應在更新子查詢的WHERE子句中使用UNION？ 還是就此而言，甚至是普通的選擇子查詢？

有沒有一種更好的方法來消除UNION這樣的查詢？

請注意，就我而言，UNION會產生相當少的記錄。

UPDATE mytable
set mytable.bla='xxx'
WHERE id IN (
    SELECT id
    FROM t1
    INNER JOIN t2 ON t2.t1_id=t1.id
    LEFT OUTER JOIN t3 ON t3.t1_id=t2.id
    WHERE t2.id IN (1,2,3) AND t3.id IS NULL
    UNION
    SELECT id FROM t4
    INNER JOIN t5 ON t5.id=t4.t5_id
    LEFT OUTER JOIN t6 ON t6.t5_id=t5.id
    WHERE t5.parent_id IN (1,2,3) AND t6.id IS NULL
);

Answer 1

將其切換為聯接：-

UPDATE mytable 
INNER JOIN
(
    SELECT id
    FROM t1
    INNER JOIN t2 ON t2.t1_id=t1.id
    LEFT OUTER JOIN t3 ON t3.t1_id=t2.id
    WHERE t2.id IN (1,2,3) 
    AND t3.id IS NULL
    UNION
    SELECT id 
    FROM t4
    INNER JOIN t5 ON t5.id=t4.t5_id
    LEFT OUTER JOIN t6 ON t6.t5_id=t5.id
    WHERE t5.parent_id IN (1,2,3) 
    AND t6.id IS NULL
) sub0
ON mytable.id = sub0.id
SET mytable.bla='xxx'