mysql连接查询左表中的空值

Question

I have the following tables

User table

user_id   name   location_id  status_id 
1         test   1               1
2         john   NULL            1
3         royi   1               1
4         mahi   2               1

Location table

location_id   location_name   location_type  status_id 
1            US                  1               1
2            UK                  1               1
3            BR                  1               1
4            IN                  2               1

I want to join the user and location table with location_id and find the null record also. I have implemented the following query but it won't working.

SELECT * 
FROM user u 
    left join location l 
        ON u.location_id = l.location_id 
WHERE 
    u.status_id = 1 
    and l.status_id = 1 
    and l.location_type = 1 
    or u.location_id IS NULL

I am expecting the result like

user_id   name   location_id  status_id location_id   location_name   location_type status_id 
1         test   1               1        1            US                  1               1
2         john   NULL            1        NULL          NULL              NULL             NULL  
3         royi   1               1        1            US                  1               1
4         mahi   2               1        2            UK                  1               1

When i change the l.location_type to 2 , the expected result like follows

user_id   name   location_id  status_id location_id   location_name   location_type status_id 
2         john   NULL            1        NULL          NULL              NULL             NULL 

Answer 1

Try this :

SELECT *
FROM user u 
    left join location l 
        ON u.location_id = l.location_id 
        and l.status_id = 1 
        and l.location_type = 1 
where 
    u.status_id = 1 

Here is the demo

You don't need to use u.location_id IS NULL because left join does not exclude users, it just adds on matched location the location of the user.

I have seen that you edited your post and changed the status_id of john from 2 to 1. Of course if John status id is 2, the query could never retrieve him (remove where u.status_id = 1 if needed)

Leave me a comment, if the expected output is wrong I'll fix it fastly.

Answer 2

I think it's because of this condition u.status_id , I guess you want to write u.status ..

Everything else seems right.. ^^

Answer 3

Since in the null records of location_id, the l.status_id and l.location_type will also be null there will be no output with your query.

You should have.

SELECT * FROM user u 
left join location l ON u.location_id = l.location_id 
WHERE u.status_id = 1 
and ((l.status_id = 1 and l.location_type = 1) or u.location_id IS NULL)

Only when you put them in brackets the OR clause will act properly and show either one of them.

I suggest this should be your query.

Answer 4

试试这个

SELECT * FROM user u left join location l ON u.location_id = l.location_id 

Answer 5

As you are using left join means you want all records from user and corresponding records from location table along with not exist records in location with null values, you can try below query:

SELECT u.*,l.* 
FROM user u 
left join location l 
ON u.location_id = l.location_id 
WHERE u.status_id = 1 and l.status_id = 1 and (l.location_type = 1 or u.location_id is null);