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Question

我有两张桌子：

Table Users

UserId     Name
1          John
2          Alice
3          Tom
4          Charles

和

Table Clients

Id         Name1          Name2      Name3
1            1              3          4
2            2              1          3

我想在哪里进行SQL查询

Clients.Id = 1

结果如下：


Id         Name1          Name2       Name3
1          John            Tom       Charles

使用INNER JOIN我可以为客户端的每一列执行此操作，但不能为整个记录（所有三列）执行此操作。

Answer 1

您需要在表users上连接表client三次。

SELECT  a.ID,
        b.name name1,
        c.name name2,
        d.name name3
FROM    clients a
        INNER JOIN users b
            ON a.name1 = b.userID
        INNER JOIN users c
            ON a.name2 = c.userID
        INNER JOIN users d
            ON a.name3 = d.userID
WHERE   a.ID = 1

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Answer 2

您需要为每个名称加入一次Users表。鉴于你有3列，可能有一个或多个可以为空，所以你更可能想要LEFT JOIN而不是INNER JOIN。

   select c.id, u1.name name1, u2.name name2, u3.name name3
     from Clients c
left join Users u1 on u1.UserId = c.name1
left join Users u2 on u2.UserId = c.name2
left join Users u3 on u3.UserId = c.name3
    where c.Id = 1

Answer 3

还有一个选项不需要过多的连接

SELECT c.Id, MAX(CASE WHEN u.UserID = c.Name1 THEN u.Name END) AS Name1,
             MAX(CASE WHEN u.UserID = c.Name2 THEN u.Name END) AS Name2,
             MAX(CASE WHEN u.UserID = c.Name3 THEN u.Name END) AS Name3
FROM dbo.Users u JOIN dbo.Clients c ON u.UserId IN (c.Name1, c.Name2, c.Name3)
WHERE c.Id = 1
GROUP BY c.Id

用另一个表中的值替换id

3 个答案:

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