Question

Assume these tables, with those fields:

History:

postID, viewerID

Posts:

postID, posterID

Now this query:

SELECT postID FROM history
WHERE postID IN (SELECT postID FROM Posts WHERE posterID IN (10,20,30)
AND viewerID=50

Will return all postIDS from posters 10,20 & 30, that viewer 50 has viewed.

What I’m after is a “group-by” that will be according to
the number of results of any posts per posterID.

For example, the results of the currently described query can have

100,200,300,300,300,100,400, 700, 500

Let’s say the first 7 results are from Poster 10 and the last 2 are from poster 20 –
So I need a result of:

posterID   viewsNum
10         7
20         2

My SQL level is intermediate, so it is not immediately clear for me how to achieve this,
sorry if there’s a ‘simple’ answer hiding inside 🙂

Answer 1

What you need here, is just a GROUP BY with an aggregate function COUNT(), and JOIN the two tables instead. Something like:

SELECT 
  PosterID, 
  COUNT(postID) viewsNum
FROM history h 
INNER JOIN Posts p ON h.postID = p.PostID
WHERE p.posterID IN (10,20,30)
  AND h.viewerID = 50
GROUP BY posterID