Question

I have one table called gallery. For each row in gallery there are several rows in the table picture. One picture belongs to one gallery. Then there is the table vote. There each row is an upvote or a downvote for a certain gallery. Here is the (simplified) structure:

gallery ( gallery_id ) picture ( picture_id, picture_gallery_ref ) vote ( vote_id, vote_value, vote_gallery_ref ) 

Now I want one query to give me the following information: All galleries with their own data fields and the number of pictures that are connected to the gallery and the sumarized value of the votes.

Here is my query, but due to the multiple joining the aggregated values are not the right ones. (At least when there is more than one row of either pictures or votes.)

SELECT    *, SUM( vote_value ) as score, COUNT( picture_id ) AS pictures FROM    gallery LEFT JOIN    vote   ON gallery_id = vote_gallery_ref LEFT JOIN    picture   ON gallery_id = picture_gallery_ref GROUP BY gallery_id 

Because I have noticed that COUNT( DISTINCT picture_id ) gives me the correct number of pictures I tried this:

( SUM( vote_value ) / GREATEST( COUNT( DISTINCT picture_id ), 1 ) ) AS score 

It works in this example, but what if there were more joins in one query?

Just want to know whether there is a better or more ‘elegant’ way this problem can be solved. Also I’d like to know whether my solution is MySQL-specific or standard SQL?

Answer 1

This quote from William of Okham applies here:

Enita non sunt multiplicanda praeter necessitatem

(Latin for ‘entities are not to be multiplied beyond necessity’).

You should reconsider why do you need this to be done in a single query? It’s true that a single query has less overhead than multiple queries, but if the nature of that single query becomes too complex, both for you to develop, and for the RDBMS to execute, then run separate queries.