Question

我第一次使用这个。

SELECT classroom .cls_id, 
   classroom.cls_name, 
   u1.users_id as users_id1,
   u2.users_id as users_id2
FROM   classroom 
   INNER JOIN clsown 
           ON clsown.cls_id = classroom .cls_id 
   INNER JOIN users AS u1 
           ON clsown.users_id = u1.users_id 
   INNER JOIN users AS u2
           ON clsown.users_id = u2.users_id

我明白了：

cls_id  cls_name  users_id1  users_id2

1 -------- room1 ------- 1 --------- 1
1 -------- room1 ------- 2 --------- 2
2 -------- room2 ------- 3 --------- 3
2 -------- room2 ------- 4 --------- 4

我想看看

cls_id  cls_name  users_id1  users_id2

1 -------- room1 ------- 1 --------- 2
2 -------- room2 ------- 3 --------- 4

我不知道如何做内连接。我做错了什么？

感谢回答。

Answer 1

您在此处注意到的查询实际上需要一个数据透视表，因为您希望获取cls_id和cls_name的唯一值，然后将用户命名为列。

我在2009年写了this article，说明如何做这样的事情。它应该有帮助

Answer 2

我假设您使用Mysql作为RDBMS，如果是，您可以使用GROUP_CONCAT()根据need显示records ，但是以不同的形式。即，此查询会将您想要的所有其他coloumns填充到由cell分隔的单个separator中。

SELECT classroom .cls_id, 
       classroom.cls_name, 
       GROUP_CONCAT(u1.users_id ORDER BY u1.users_id SEPARATOR ', ') Users 
FROM   classroom 
       INNER JOIN clsown 
               ON clsown.cls_id = classroom .cls_id 
       INNER JOIN users AS u1 
               ON clsown.users_id = u1.users_id
       Group by classroom .cls_id

上述查询将返回如下记录，

cls_id  cls_name    users

  1      room1       1,2

  2      room2       3,4

编辑： Reference for achieving the above result in Sql server

Answer 3

由于您使用的是SQL Server，因此有几种方法可以转移数据。

您可以使用带有CASE表达式的聚合函数：

select cls_id,
    cls_name,
    MAX(case when rn = 1 then users_id end) user_id1,
    MAX(case when rn = 2 then users_id end) user_id2
from
(
    SELECT cr.cls_id, 
        cr.cls_name, 
        u1.users_id,
        ROW_NUMBER() over(partition by cr.cls_id order by u1.users_id) rn
    FROM classroom cr
    INNER JOIN clsown co
        ON co.cls_id = cr.cls_id 
    INNER JOIN users AS u1 
        ON co.users_id = u1.users_id 
) d
group by cls_id, cls_name;

或者您可以使用PIVOT功能：

select cls_id,
    cls_name,
    user_id1, 
    user_id2
from
(
    SELECT cr .cls_id, 
        cr.cls_name, 
        u1.users_id,
        'user_id'+cast(ROW_NUMBER() over(partition by cr.cls_id order by u1.users_id) as varchar2)) col
    FROM classroom cr
    INNER JOIN clsown co
        ON co.cls_id = cr.cls_id 
    INNER JOIN users AS u1 
        ON co.users_id = u1.users_id 
) d
pivot
(
    max(users_id)
    for col in (user_id1, user_id2)
) piv;

选择内连接表id vb

3 个答案: