select inner join column2 NULL [closed]

问题

This question is unlikely to help any future visitors; it is only relevant to a small geographic area, a specific moment in time, or an extraordinarily narrow situation that is not generally applicable to the worldwide audience of the internet. For help making this question more broadly applicable, visit the help center.

Closed 6 years ago.

table clsown

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;

i get this

cls_id cls_name users_id1 users_id2
  1     room1       1        NULL

i want see

cls_id cls_name users_id1 users_id2
  1     room1       1         2

回答1:

Your code has a and u1.users_id =1 in it, which is why you're getting the results you're seeing. bluefeet's SQL Fiddle shows the results without that and it looks like it works fine.

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

回答2:

Remove the and ul.users_id = 1 from the on clause:

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;

来源：https://stackoverflow.com/questions/15664926/select-inner-join-column2-null