Get the id_user_app if not exists in id_user_rq

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Question :

I’m building a application and have this table for users relationships. I need to build some queries like for example:

  • Who invited me as a friend?
  • Who I invited to be my friend?
  • Who I accept as friend?

And need some help because for me are complex queries :(. Also I don’t know if open trhead for each of the question or leave all them here in this topic, what’s the best?

For the first question: I have this table structure: http://www.dropmocks.com/mBjNvF and need to get the id_user_app if not exists in id_user_rq as the title said. I try this query:

SELECT DISTINCT A.*, p.*, u.email FROM default_relations_users A 
    LEFT JOIN default_relations_users B ON (A.id_user_rq = B.id_user_ap
    AND A.id_user_ap = B.id_user_rq AND A.id_user_rq = 1)
    LEFT JOIN default_users u ON (u.id = A.id_user_rq) 
    LEFT JOIN default_profiles p ON (p.user_id = A.id_user_rq)
    GROUP BY A.id_user_rq

But it’s not working, any advice or help?

Answer :

SELECT DISTINCT A.*, p.*, u.email FROM
              (SELECT * FROM default_relation_users WHERE id_user_rq = 1) A
              LEFT JOIN
              (SELECT * FROM default_relation_users WHERE id_user_ap = 1) B
              ON (A.id_user_rq=B.id_user_ap AND B.id_user_rq=A.id_user_ap)
              LEFT JOIN default_users u ON (u.id = A.id_user_rq) 
              LEFT JOIN default_profiles p ON (p.user_id = A.id_user_rq)
              WHERE B.id_user_ap IS NULL
              GROUP BY A.id_user_rq

I changed A and B to be subqueries using 1 for id_user_rq and id_user_ap

If you want the opposite, just reverse A and B subqueries to something like this:

SELECT DISTINCT A.*, p.*, u.email FROM
              (SELECT * FROM default_relation_users WHERE id_user_ap = 1) A
              LEFT JOIN
              (SELECT * FROM default_relation_users WHERE id_user_rq = 1) B
              ON (A.id_user_rq=B.id_user_ap AND B.id_user_rq=A.id_user_ap)
              LEFT JOIN default_users u ON (u.id = A.id_user_rq) 
              LEFT JOIN default_profiles p ON (p.user_id = A.id_user_rq)
              WHERE B.id_user_rq IS NULL
              GROUP BY A.id_user_rq

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