Intro to Having

The Having Clause lets us filter our data on some condition dependent on the output of some aggregation function. It is similar to a WHERE clause in that we a specifying a search condition on our query.

Questions

We could use it to answer questions like:

  • Which albums in our database have more than 10 songs?
  • Which email addresses appear more than once in our table?

Example

Let’s try to answer the first question.

music.AlbumTrack

In our music database, the relation between song and album is represented by the table music.AlbumTrack. Here are the first five rows:

AlbumID and SongID are foreign keys that relate to a row in music.Album and music.Song respectively. This table is comprised of unique AlbumID and SongID pairs. This is because the participation between a album and a song is many-to-many - A song can appear on many albums and an album can have many songs.

What this means for our usage is that AlbumID is will have repetitions. Each repetition represents a another song on that album.

In the below code, we aggregate by the number of songs on each album and return only those albums featuring greater than 10 songs.

SQL Having

SELECT
	AlbumID
FROM music.AlbumTrack
GROUP by
	AlbumID	
HAVING COUNT(*) > 5;

Output

Odd that we only have just three albums in the database with more than 10 songs!

This is what happens when you use semi-randomly generated dummy data.

Further analysis

We could now ask:

  • How many songs do each of these albums have?
  • What are the actual names of these albums?
  • Who are the main artists for these albums?

SQL

For the first question, we simpply add COUNT(*) to the SELECT. The last two questions can be answered if we join to music.Album.

This table is normalized which is important because we will be using INNER JOIN. Therefore we don’t need to worry about the cartesian product returning more rows than we started with!

SELECT
	music.Album.AlbumId
,	title
,	MainArtist
,	COUNT(*) AS SongCount
FROM	music.AlbumTrack
INNER JOIN	music.Album
ON music.AlbumTrack.AlbumID = music.Album.AlbumID
GROUP BY 
	music.Album.AlbumId
,	title
,	MainArtist
HAVING COUNT(*) > 10;

Output

Now we know the album titles, main artist and song count. These albums lack a main artist, which means they are compilation albums. By sheer coincidence, they each have 15 songs.