Question

I have one table and I need to check if two users, for whom I have the IDs (e.g. 20 and 21) share the same course, just true or false.

Table: jos_gj_users
Columns: id_user, id_group

Data Example: (20; 4)
              (20; 5)
              (20; 6)
              (21; 6)
              (21; 7)

The data above shows that user 20 and user 21 share the course 6 but how do I get this with SQL just by entering the IDs and without looping through the results with PHP?

Answer 1

Try a self-join:

SELECT T1.id_group
FROM jos_gj_users T1
JOIN jos_gj_users T2
ON T1.id_group = T2.id_group
WHERE T1.id_user = 20
AND T2.id_user = 21

To just get a “true or false” result you can check from the client to see if at least one row exists in the result set rather than fetching the entire results.

Alternatively you can do it in SQL by wrapping the above query in another SELECT that uses EXISTS:

SELECT CASE WHEN EXISTS
(
    SELECT T1.id_group
    FROM jos_gj_users T1
    JOIN jos_gj_users T2
    ON T1.id_group = T2.id_group
    WHERE T1.id_user = 20
    AND T2.id_user = 21
) THEN 1 ELSE 0 END AS result

This query returns either 0 (false) or 1 (true).