将相关子查询重写为 CROSS APPLY

Question

所以我正在尝试使用 CROSS APPLY 但似乎无法正确使用它。

我有一些类似这样的查询：

SELECT COL1, COL2
FROM dbo.tableA AS A
WHERE COL3 = (SELECT MAX(COL4)
              FROM dbo.tableA AS B
              WHERE A.COL1 = B.COL1) AS SUB

我试试这个：

SELECT COL1, COL2
FROM dbo.tableA AS A
CROSS APPLY (SELECT MAX(COL4) AS MAX_DATE
              FROM TABLEA AS B
              WHERE A.COL1 = B.COL1) AS SUB

但是当我使用 CROSS APPLY 时，我总是 return 多行。我的错误在哪里？

Answer 1

您在第二个查询中遗漏了 WHERE，如果您希望这两个查询相同：

SELECT COL1, COL2
FROM dbo.tableA AS A
CROSS APPLY (SELECT MAX(COL4) AS MAX_DATE
              FROM TABLEA AS B
              WHERE A.COL1 = B.COL1) AS SUB 
WHERE A.COL3 = SUB.MAX_DATE;

Answer 2

除了 APPLY 重写之外，因为这是一个自连接，所以它也可以通过 MAX window 聚合来完成：

SELECT COL1, COL2
FROM (
    SELECT *,
        MAX(COL4) OVER (PARTITION BY COL1) AS mx
    FROM dbo.tableA
) AS A
WHERE COL3 = mx

将相关子查询重写为 CROSS APPLY

Rewrite correlated subquery to CROSS APPLY

tsql

sql-server

subquery

cross-apply

correlated-subquery