连接 2 个表，在另一个表上选择最高值

Question

我正在尝试加入 2 个表，但仅从表 B 中选择 id 最大且表 A 和 B 之间有 2 列匹配的值。

我已经尝试过了，但由于表 2 的值最高，它只选择了 1 行，但我需要显示整个表 A。

WITH CTE AS 
(
    SELECT DocNum, Status 
    FROM TableB 
    WHERE Id = (SELECT MAX(Id) FROM  TableB)
)
SELECT  TableA.DocNum, DocTitle, cte.Status
FROM TableA
INNER JOIN cte 
ON TableA.DocNum = cte.DocNum

下面的例子：

Table A
    Id DocNum  DocTitle
    1  ST-0001 SomeTitle
    2  ST-0002 SomeTitle
    3  ST-0003 SomeTitle
    4  ST-0004 SomeTitle
    5  ST-0005 SomeTitle

Table B
    Id DocNum Status
    1  ST-0001 Incomplete
    2  ST-0001 Complete
    3  ST-0002 Complete
    4  ST-0003 Incomplete
    5  ST-0004 Incomplete
    6  ST-0004 Complete 
    7  ST-0005 Incomplete   

Result
    DocNum DocTitle Status
    ST-0001 SomeTitle Complete  
    ST-0002 SomeTitle Complete  
    ST-0003 SomeTitle Incomplete  
    ST-0004 SomeTitle Complete  
    ST-0005 SomeTitle Incomplete

Answer 1

您可以使用带有 Row_number 的 CTE 从 tableB 中仅获取每个 docNum 具有最大 id 的行，然后加入该 cte。

首先，创建并填充示例表（请在您以后的问题中保存我们这一步）：

DECLARE @A As Table
(
    Id int,
    DocNum char(7),
    DocTitle varchar(20)
);

DECLARE @B As Table
(
    Id int,
    DocNum char(7),
    [Status] varchar(20)
);

INSERT INTO @A VALUES
(1, 'ST-0001', 'SomeTitle'),
(2, 'ST-0002', 'SomeTitle'),
(3, 'ST-0003', 'SomeTitle'),
(4, 'ST-0004', 'SomeTitle'),
(5, 'ST-0005', 'SomeTitle');

INSERT INTO @B VALUES
(1, 'ST-0001', 'Incomplete'),
(2, 'ST-0001', 'Complete'),
(3, 'ST-0002', 'Complete'),
(4, 'ST-0003', 'Incomplete'),
(5, 'ST-0004', 'Incomplete'),
(6, 'ST-0004', 'Complete'),
(7, 'ST-0005', 'Incomplete');

然后，使用 tableB 中的 row_number 创建 cte：

;WITH CTE AS
(
    SELECT  Id, 
            DocNum, 
            [Status],
            ROW_NUMBER() OVER(PARTITION BY DocNum ORDER BY Id DESC) As Rn
    FROM @B
)

查询：

SELECT A.DocNum, A.DocTitle, CTE.[Status]
FROM @A A 
INNER JOIN CTE ON A.DocNum = CTE.DocNum
WHERE CTE.rn = 1

结果：

DocNum      DocTitle    Status
ST-0001     SomeTitle   Complete  
ST-0002     SomeTitle   Complete  
ST-0003     SomeTitle   Incomplete  
ST-0004     SomeTitle   Complete  
ST-0005     SomeTitle   Incomplete

You can see a live demo on rextester.