sql – GROUP BY一列;为另一个选择任意值

我试图为每个用户选择一行.我不关心我得到的图像.此查询适用于 MySQL,但不适用于SQL Server：

SELECT user.id,(images.path + images.name) as 'image_path'
FROM users
JOIN images ON images.user_id = users.id
GROUP BY users.id

解决方法

使用AdventureWorks sample database来说明,以下查询都为每个ProductID从Transaction History表中选择一个TransactionType和ReferenceOrderID：

使用MIN / MAX聚合

SELECT
    p.ProductID,MIN(th.TransactionType + STR(th.ReferenceOrderID,11))
FROM Production.Product AS p
INNER JOIN Production.TransactionHistory AS th ON
    th.ProductID = p.ProductID
GROUP BY
    p.ProductID;

使用ROW_NUMBER

WITH x AS 
(
    SELECT 
        th.ProductID,th.TransactionType,th.ReferenceOrderID,rn = ROW_NUMBER() OVER (PARTITION BY th.ProductID ORDER BY (SELECT NULL))
    FROM Production.TransactionHistory AS th
)
SELECT
    p.ProductID,x.TransactionType,x.ReferenceOrderID
FROM Production.Product AS p
INNER JOIN x ON x.ProductID = p.ProductID
WHERE
    x.rn = 1
OPTION (MAXDOP 1);

使用仅限内部的ANY聚合

SELECT
    q.ProductID,q.TransactionType,q.ReferenceOrderID 
FROM 
(
    SELECT 
        p.ProductID,rn = ROW_NUMBER() OVER (
            PARTITION BY p.ProductID 
            ORDER BY p.ProductID)
    FROM Production.Product AS p
    JOIN Production.TransactionHistory AS th ON p.ProductID = th.ProductID
) AS q
WHERE
    q.rn = 1;

有关任何汇总的详细信息,请参阅this blog post.

使用具有非确定性TOP的相关子查询

SELECT p.ProductID,(
    -- No ORDER BY,so could be any row
    SELECT TOP (1) 
        th.TransactionType + STR( th.ReferenceOrderID,11)
    FROM Production.TransactionHistory AS th WITH (FORCESEEK) 
    WHERE
        th.ProductID = p.ProductID
    )
FROM Production.Product AS p;

使用CROSS APPLY和TOP(1)

上一个查询需要连接,并为没有事务历史记录的产品返回NULL.使用CROSS APPLY和TOP解决了这两个问题：

SELECT
    p.Name,ca.TransactionType,ca.ReferenceOrderID
FROM Production.Product AS p
CROSS APPLY
(
    SELECT TOP (1) 
        th.TransactionType,th.ReferenceOrderID
    FROM Production.TransactionHistory AS th WITH (FORCESEEK) 
    WHERE 
        th.ProductID = p.ProductID
) AS ca;

通过最佳索引,并且如果每个用户通常具有许多图像,则APPLY可能是最有效的.

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