Question

I am trying to learn about MYSQL Joins but am having trouble figuring this one out. I want to get all rows in table1 where the userID is in a relationship with the given user.

table2 contains user relationships so the userID can either be in iduser1 field or iduser2 field
I have the following query that gives me the correct results:

set @userID = 91;
SELECT * FROM table1
    WHERE (iduser IN (SELECT iduser1 FROM table2 WHERE iduser2 = @userID)
        OR (iduser IN (SELECT iduser2 FROM table2 WHERE iduser1 = @userID)))

table1: 
    iduser FK

table2:
    iduser1 FK
    iduser2 FK

I have been told that nested queries in MySQL have a bad reputation when it comes to performance and I am sure I could probably do this same query using JOINS somehow but I just cant figure it out especially as there is an OR statement because the table1.iduser can be in either table2.iduser1 OR table2.iduser2

How can I join the same table twice?

Answer 1

Given this expression:

set @userID = 91;
SELECT * FROM table1
    WHERE (iduser IN (SELECT iduser1 FROM table2 WHERE iduser2 = @userID)
        OR (iduser IN (SELECT iduser2 FROM table2 WHERE iduser1 = @userID)))

Using EXISTS, you can reduce that two subqueries into following:

set @userID = 91;
SELECT * FROM table1 x
WHERE EXISTS(
    select * from table2 z
    where (z.user2 = @userID and iduser1 = x.iduser)
       OR (z.user1 = @userID and iduser2 = x.iduser)
)

The OR might slowdown your query, it doesn’t necessarily short-circuit in RDBMS. In that case, you should try to short-circuit it by using CASE WHEN. I had a query that took 5 seconds to execute when using OR, but took sub-zero second when I translated it to CASE WHEN:

set @userID = 91;
SELECT * FROM table1 x
WHERE EXISTS(
    select * from table2 z
    where 
       CASE WHEN z.user2 = @userID THEN
           IF(z.iduser1 = x.iduser, 1, 0)
       CASE WHEN z.user1 = @userID THEN 
           IF(z.iduser2 = x.iduser, 1, 0)
       END = 1
)

Boolean expression automatically cast to integer(either 0(false) or 1(true)), so you can also do it like this:

set @userID = 91;
SELECT * FROM table1 x
WHERE EXISTS(
    select * from table2 z
    where 
       CASE WHEN z.user2 = @userID THEN
           z.iduser1 = x.iduser
       CASE WHEN z.user1 = @userID THEN 
           z.iduser2 = x.iduser
       END = 1
)

Or this:

set @userID = 91;
SELECT * FROM table1 x
WHERE EXISTS(
    select * from table2 z
    where 
       CASE WHEN z.user2 = @userID THEN
           z.iduser1 = x.iduser
       CASE WHEN z.user1 = @userID THEN 
           z.iduser2 = x.iduser
       END 
)

---

If you are not using MySQL, you shall do it like this:

set @userID = 91;
SELECT * FROM table1 x
WHERE EXISTS(
    select * from table2 z
    where 
       CASE WHEN z.user2 = @userID THEN
           CASE WHEN z.iduser1 = x.iduser THEN 1 END
       CASE WHEN z.user1 = @userID THEN 
           CASE WHEN z.iduser2 = x.iduser THEN 1 END
       END = 1
)