Question

I am have two tables which I have left joined like so below to show me the pads by id added in descending order which works fine.

SELECT p.* FROM ruj_users_pad 
AS p LEFT JOIN ruj_users
AS u ON p.user_id = u.id 
WHERE u.status !=0 
AND 1 
AND p.status = 1 
GROUP BY p.added_date
DESC LIMIT 0, 20

However, now I would like to retrieve a third table which also as an ‘added_date’ column combine it with the previous query to show the new descending order. The data from the third table is a generated from a button when users click to favorite the current item on the pad.

Here is what I have but it is not working.

SELECT p.*,f.added_date FROM ruj_users_pad
AS p LEFT JOIN ruj_users 
AS u ON b.user_id=u.id 
LEFT JOIN ruj_users_fave 
AS f ON f.brag_id = u.id 
WHERE u.status !=0 
AND 1 
AND p.status = 1 
GROUP BY f.added_date DESC, b.added_date DESC 
LIMIT 0, 20

The result returns the same as the first result. I don’t understand what could be wrong. I would like the result to take into consideration that there is an entry in the ruj_users_fave and combine it with the first result to bring the favorited pad to the top.

Help is greatly appreciated.

Answer 1

Here is a solution to your problem using UNION to merge two subqueries. It is not the full query of three tables but only the essence with two tables to demonstrate the concept. I use a table pad and a table likes. The first subquery selects the pad’s id and its most recent “like” using MAX together with GROUP BY id. Note that I rename the column MAX(liked) to timestamp.

SELECT pad.id, MAX(liked) AS timestamp FROM
pad INNER JOIN likes WHERE likes.pad_id = pad.id
GROUP BY pad.id

The other subquery selects the pad ids that were not liked yet, i.e. no record in the likes table exists for that specific pad id. In this case, you mention that the pad’s creation date should be used for determining the order of records returned. Thus, we rename added to timestamp.

SELECT id, added AS timestamp FROM pad
WHERE NOT EXISTS
  (SELECT liked FROM likes WHERE likes.pad_id = pad.id)

We have two subqueries now that return the pad’s id and the timestamp so we can combine them to a single result set using UNION, and then ORDER it BY timestamp. The combined query for you looks like this.

SELECT combined.id, combined.timestamp FROM
(
  SELECT pad.id, MAX(liked) AS timestamp FROM
    pad INNER JOIN likes WHERE likes.pad_id = pad.id
    GROUP BY pad.id
  UNION
  SELECT id, added AS timestamp FROM pad
    WHERE NOT EXISTS
      (SELECT liked FROM likes WHERE likes.pad_id = pad.id)
)
combined ORDER BY combined.timestamp DESC;