Question

I have the tables users, locations and a join table. The join table also has a rank value for each individual at that location, because those individuals have higher ranks at certain locations than at other locations.

users
=========
id | name
=========
 1 | john
 2 | bob
 3 | alex

locations
===============
id | name
===============
 1 | san diego
 2 | dallas
 3 | new york
 4 | denver

join_users_locations
==============================
user_id | location_id | rank
==============================
      2 |           1 | 4
      2 |           2 | 3
      2 |           4 | 2
      3 |           1 | 2
      3 |           2 | 4

You can see that in the join table a user is only listed if they have a rank at that location. They are not even in the table if their rank is zero/null.

I’m wanting to get query results like this:

name |  location | rank 
=======================
 bob | san diego | 4
 bob |    dallas | 3
 bob |  new york | 0
 bob |    denver | 2

As you can see, I want all locations to be in the list, even ones that the user isn’t joined with (just give those fields a rank value of zero).

It’s easy to get the list of locations and ranks for locations he/she is joined with:

SELECT
    u.name,
FROM users u
LEFT JOIN join_users_locations jul ON jul.user_id = u.id
LEFT JOIN locations l ON l.id = jul.location_id
WHERE u.id = 2

But that only lists what locations the user is joined with. I want to see the non-joined locations as well.

I know this is possible, but I’m not sure what the solution is. I’ve tried some different types of JOINs with various results.

SQL FIDDLE HERE

Answer 1

You can do this with a CROSS JOIN from users to locations.

SELECT
    u.name,
    l.name AS location,
    IFNULL(jul.rank, 0) AS rank
FROM
    users u CROSS JOIN
    locations l LEFT JOIN
    join_users_locations jul 
        ON jul.user_id = u.ID 
        AND jul.location_id = l.id
WHERE u.id = 2