Question

此请求有效：

有两个连续的FROM。当我删除第一个时，为什么不起作用？

DELETE FROM TableA
FROM TableA dim
LEFT OUTER JOIN (
    SELECT DISTINCT ColA
    FROM TableB
    UNION ALL
    SELECT DISTINCT ColA
    FROM tableC

) A ON A.ColA= dim.ColA
WHERE A.ColA IS NULL

编辑我认为这是不正确的，这是正确的：

 DELETE 
    FROM TableA dim
    LEFT OUTER JOIN (
        SELECT DISTINCT ColA
        FROM TableB
        UNION ALL
        SELECT DISTINCT ColA
        FROM tableC

    ) A ON A.ColA= dim.ColA
    WHERE A.ColA IS NULL

谢谢

Answer 1

这是因为表联接。 SQL引擎需要知道要从哪个源中删除，而奇数FROM FROM语法可以完成此任务。

您拥有的语句在功能上等效于此语句，除了将表别名替换为第一个FROM子句之外，该语句完全相同：

DELETE dim
FROM TableA dim
LEFT OUTER JOIN (
    SELECT DISTINCT ColA
    FROM TableB
    UNION ALL
    SELECT DISTINCT ColA
    FROM tableC

) A ON A.ColA= dim.ColA
WHERE A.ColA IS NULL

Answer 2

您可以使用NOT EXISTS：

DELETE dim
FROM TableA dim
WHERE NOT EXISTS (SELECT 1 FROM TableB B WHERE B.ColA = DIM.ColA) OR
      NOT EXISTS (SELECT 1 FROM TableC C WHERE C.ColA = DIM.ColA);

您的delete语句有两个不正确的from子句，它在delete语句之后应该只有一个立即数：

DELETE dim
FROM TableA dim LEFT OUTER JOIN 
    (SELECT ColA
     FROM TableB
     UNION 
     SELECT ColA
     FROM tableC
    ) A 
    ON A.ColA= dim.ColA
WHERE A.ColA IS NULL;

此外，如果要输入DISTINCT，然后使用UNION ALL，那么UNIQUE和UNION就是多余的。

SQL DELETE STATEMENT说明

2 个答案: