Question

i have many to many database and i’m there is a query that i just got stuck with and cant do it.

i have 4 tables Artists, Tracks, Albums, Clips

now i’m on the artist page so i need to get them by the artist page, i already got all of them, but not the way i want them.

because some tracks, albums, clips belong to other artists as well (duet) and i need to display their name.

but the problem is that i’m selecting using the artist id so my GROUPC_CONCAT function wont work here is the query that gets the artist albums.

   SELECT  al.album_name, GROUP_CONCAT(a.artist_name SEPARATOR ', ') AS 'artist_name'

   FROM 
     Albums al
   LEFT JOIN 
     ArtistAlbums art ON art.album_id = al.album_id
   LEFT JOIN 
     Artists a on a.artist_id = art.artist_id
   WHERE 
     a.artist_id = 10
   GROUP BY 
     al.album_id

one of the albums have two artists attached to it, but it does not get the other artist name.

when i select by the album_id i get the two artists.

please note that i’m new to mysql and i did not find any answers on this particular problem almost no resources on many-to-many querying.

how can i tackle this problem.?

any resources or books on many-to-many that show how to deal with the database on the application layer will be much appreciated,
thanks in advance.

Answer 1

Think of table aliases as really being row aliases. That is, for purposes of expressions in the WHERE clause and the select-list, the alias refers to a single row at a time.

Since you’ve created a condition such that a.artist_id = 10, then only rows for that artist match the condition. What you really want is to match all artists on an album given that one artist is artist_id = 10.

For that, you need another join, so that the row where artist_id = 10 is matched to all the rows for that respective album.

SELECT  al.album_name, GROUP_CONCAT(a2.artist_name SEPARATOR ', ') AS `artist_name`
FROM 
  Albums al
INNER JOIN 
  ArtistAlbums art ON art.album_id = al.album_id
INNER JOIN 
  Artists a on a.artist_id = art.artist_id
INNER JOIN 
  ArtistAlbums art2 ON art2.album_id = al.album_id
INNER JOIN 
  Artists a2 on a2.artist_id = art2.artist_id
WHERE 
  a.artist_id = 10
GROUP BY 
  al.album_id

P.S.: I’ve also replaced your use of LEFT JOIN with INNER JOIN. There’s no reason you needed LEFT JOIN, since you’re explicitly looking for albums that have a matching artist, not all albums whether or not it has artist 10. Review the meaning of different types of join.

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Re your followup question:

I don’t know of a book specifically about many-to-many queries. I’d recommend books like:

• SQL Antipatterns Volume 1: Avoiding the Pitfalls of Database Programming, which is my own book and it does cover many-to-many tables.
• SQL and Relational Theory to understand joins better.
• Joe Celko’s SQL Programming Style, which imho is Joe Celko’s best book.