In our article Exploring a Data Set in SQL we discovered a data set related to music: the Chinook sample database.

Our discovery led us to find albums containing tracks of multiple genres, and for the analytics we were then pursuing, we wanted to clean the data set and assign a single genre per album. We did that in SQL of course, and didn’t actually edit the data.

Finding the most frequent input value in a group is a job for the mode() WITHIN GROUP (ORDER BY sort_expression) Ordered-Set Aggregate Function, as documented in the PostgreSQL page about Aggregate Functions.

For more about the Mode function name, read the Wikipedia article that explain the Mathematics background behind it at Mode (statistics).

Multi-genres albums

In order to dive into the data, let’s have a look at it from the genre point of view. As we saw in the Chinook database model a given album is always edited by a single artist, but might contain several different genres, one per track. Do we have multi-genre albums?

  select title as album,
             distinct, ', '
             order by
         as genres
    from      track
         join genre using(genreid)
         join album using (albumid)
group by title
  having count(distinct > 1;

We have the following list of multi-genre albums in our collection:

             album              │                   genres                    
 Battlestar Galactica, Season 3 │ Sci Fi & Fantasy, Science Fiction, TV Shows
 Greatest Hits                  │ Metal, Reggae, Rock
 Heroes, Season 1               │ Drama, TV Shows
 LOST, Season 4                 │ Drama, TV Shows
 Live After Death               │ Heavy Metal, Metal
 Lost, Season 2                 │ Drama, TV Shows
 Lost, Season 3                 │ Drama, TV Shows
 Rock In Rio [CD2]              │ Metal, Rock
 The Number of The Beast        │ Metal, Rock
 The Office, Season 3           │ Comedy, TV Shows
 Unplugged                      │ Blues, Latin
(11 rows)

Selecting a single genre per album

In the previous article, we manually counted the number of tracks of each genre per album and selected the most frequent genre: the one with the greatest number of tracks.

SQL provides an Ordered-Set Aggregate Function that does just that:

with multi_genre_albums
 as ( 
    select albumid
      from track
  group by albumid
    having count(distinct genreid) > 1
 select as artist,
        album.title as title, as genre,
        mode() within group (order by genreid)
   from multi_genre_albums
        join genre using(genreid)
        join album using(albumid)
        join artist using (artistid);

In this query we limit our processing to albums containing tracks of several genres, thanks to the multi_genre_albums CTE, where we use a simple GROUP BY and HAVING clause. We then use the result of the CTE and fetch supplementary information about the albums, and use the mode() Set-Ordered Aggregate Function to retain only the most frequent genreid:

        artist        │             title              │      genre      
 Eric Clapton         │ Unplugged                      │ Latin
 Iron Maiden          │ The Number of The Beast        │ Metal
 Iron Maiden          │ Rock In Rio [CD2]              │ Rock
 Iron Maiden          │ Live After Death               │ Metal
 Lenny Kravitz        │ Greatest Hits                  │ Rock
 Battlestar Galactica │ Battlestar Galactica, Season 3 │ Science Fiction
 Heroes               │ Heroes, Season 1               │ Drama
 Lost                 │ LOST, Season 4                 │ Drama
 Lost                 │ Lost, Season 2                 │ TV Shows
 Lost                 │ Lost, Season 3                 │ Drama
 The Office           │ The Office, Season 3           │ Comedy
(11 rows)

Full documentation for the mode() function says:

returns the most frequent input value (arbitrarily choosing the first one if there are multiple equally-frequent results)

Use it next time you want to select only the most-frequent element of a group in your result-set.