Question

I can’t seem to figure out to make join statements.

select usrs.*
from
    students s
    join system_users usrs on
        s.father=usrs.user_id
        OR s.mother=usrs.user_id
        OR s.guardian=usrs.user_id
where s.user_id = '201209083'

I have this query, and it returns :

+------------+---------+--------+--------+
| user_id    | contact | status | rights |
+------------+---------+--------+--------+
| f201209083 |    4093 | active | NULL   |
| g201209083 |    4095 | active | NULL   |
| m201209083 |    4094 | active | NULL   |
+------------+---------+--------+--------+

Now what I need is to use the information of the contact column to retrieve more information and join it with this table:

select
    *
from
    address
where
    contact_id = (contact column);

I need help with implementing it in one query.

P.S. Can someone tell me what this technique is called? I cannot find information on this by just searching for JOINs. Or a link to learn would be even better.

Answer 1

You could take that current statement as a sub-query and join it as such:

SELECT * FROM address 
JOIN (
    SELECT 
        usrs.* 
    FROM 
        students s 
        JOIN system_users usrs ON s.father=usrs.user_id OR s.mother=usrs.user_id OR s.guardian=usrs.user_id 
    WHERE 
        s.user_id = '201209083'
) AS temp 
ON contact_id = (contact column);

That should give you the original results combined with the results from the address table.