Question

I’ve been working on some MySQL Joins but this one has got me a little confused.
I have 3 tables, friends, posts and activity.

Tables posts and activity both have a field userid which represents the user that the record belongs to. friends contains the fields userid and friendid to represent two users being friends.

If possible, I would like to design a query which will display the records in posts AND activity from a user which is friends with the logged in user.

I have tried the following query:

SELECT p.*, a.*
    FROM posts AS p, activity AS a
    INNER JOIN friends f ON (p.userid = f.friendid)
    WHERE f.userid = '(Logged In User ID)'

However, when I run the above query, I get the following error:

Unknown column 'p.userid' in 'on clause'

Any help would be much appreciated 🙂

Answer 1

You probably need a UNION. The following will work if the two tables, posts and activity have the same number of columns (in same order and similar type). If not, some adjustment is needed in the SELECT lists:

SELECT p.*
    FROM posts AS p
        INNER JOIN friends f ON p.userid = f.friendid
    WHERE f.userid = '(Logged In User ID)'
UNION ALL
SELECT a.*
    FROM activity AS a
        INNER JOIN friends f ON a.userid = f.friendid
    WHERE f.userid = '(Logged In User ID)'

If the two tables have different fields, then you should adjust the 2 select lists, with something like:

SELECT p.id
     , p.userid
     , p.post_text     AS text
     , p.post_date     AS date
     , NULL            AS action
    FROM posts AS p
    ...
UNION ALL
SELECT a.id
     , a.userid
     , NULL            AS text 
     , a.activity_date AS date
     , a.action        AS action
    FROM activity AS a
    ...