选择内部连接column2 NULL [关闭]

Question

表关闭

id cls_id users_id

1----1---------1

2----1---------2

sql

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 and u1.users_id =1
) d
group by cls_id, cls_name;

我明白了

cls_id cls_name users_id1 users_id2
  1     room1       1        NULL

我想看看

cls_id cls_name users_id1 users_id2
  1     room1       1         2

Answer 1

您的代码中包含and u1.users_id =1，这就是您获得所见结果的原因。 bluefeet 的 SQL Fiddle 显示没有它的结果，看起来它工作正常。

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;

SQL Fiddle Demo

Answer 2

从on 子句中删除and ul.users_id = 1：

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;