左加入具有特定条件的子查询

Question

我有两个表要加入

table1
----------------------------
Id         Name         Num
123X       Apple        17

table2
-------------------------------------------------
id           EndDt         SomeVal      
123X         10/1/2021     xxx
123X         3/1/2022      yyy

我正在尝试从 table1 a 和 LEFT JOIN table2 b on a.id = b.id Select - 但是，我只想 select 表 2 中的 ID MAX(EndDt)

Select a.*, b.SomeVal
from table1 a 
    LEFT OUTER JOIN table2 b on a.id=b.id // and b.MAX(EndDt)

这样的事情可行吗？

Answer 1

我建议使用窗口 ROW_NUMBER 函数首先获取最大值 table2，然后加入该子查询。

;WITH cte AS (
    SELECT *, [Row] = ROW_NUMBER() OVER (PARTITION BY b.Id ORDER BY b.EndDt DESC)
    FROM table2 b
)
SELECT a.*, cte.SomeVal
FROM table1 a
LEFT JOIN cte ON a.id = cte.id AND cte.[Row] = 1

Answer 2

对于单个值，使用相关子查询：

SELECT
    a.id,
    a.name,
    a.num,
    (
        SELECT TOP 1 SomeValue 
        FROM table2 As b 
        WHERE b.id = a.id 
        ORDER BY b.EndDt DESC
    ) As SomeVal
FROM
    table1 a

Answer 3

有几种方法可以做到这一点。不过，我对您的数据做了一些假设。

对子查询使用 LEFT JOIN：

SELECT T1.*,
       sq.SomeVal
FROM dbo.Table1 T1
     LEFT JOIN (SELECT ROW_NUMBER() OVER (PARTITION BY t2.Id ORDER BY t2.EndDt DESC) AS RN,
                       t2.Id,
                       t2.SomeVal
                FROM dbo.Table2 T2) sq ON T1.Id = T2.Id
                                      AND T2.RN = 1;

使用APPLY和TOP：

SELECT T1.*,
       sq.SomeVal
FROM dbo.Table1 T1
     OUTER APPLY (SELECT TOP (1)
                         t2.Id,
                         t2.SomeVal
                  FROM dbo.Table2 T2
                  WHERE T2.Id = T1.Id
                  ORDER BY T2.EndDt DESC) sq;

使用 CTE 并获取每组的“前 1”行：

WITH CTE AS(
    SELECT T1.*,
           T2.SomeVal,
           ROW_NUMBER() OVER (PARTITION BY T1.ID ORDER BY T2.MaxDt DESC) AS RN
    FROM dbo.Table1 T1
         LEFT JOIN dbo.Table2 T2 ON T1.Id = T2.Id)
SELECT *
FROM CT
WHERE RN = 1;

使用TOP (1) WITH TIES：

SELECT TOP (1) WITH TIES
       T1.*,
       T2.SomeVal
FROM dbo.Table1 T1
     LEFT JOIN dbo.Table2 T2 ON T1.Id = T2.Id
ORDER BY ROW_NUMBER() OVER (PARTITION BY T1.ID ORDER BY T2.MaxDt DESC) ASC;

请注意，如果 ID 在 table Table1 中不是唯一的（因此我对您的数据做出了假设），选项 3 和 4 将不会按预期工作。

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