Joins: Aliases

AS after a column or table precedes an alias.

SELECT
	trk.AlbumID
,	alb.Title AS AlbumTitle 
,	trk.SongID
,	trk.TrackNo
,	sng.Name AS SongTitle
FROM		AlbumTrack 	AS trk
INNER JOIN	Album 		AS alb
ON trk.AlbumID = alb.AlbumID
INNER JOIN	Song 		AS sng
ON trk.SongID = sng.SongID;

Table Aliases

In the above example, we can see a few Table Aliases, these being trk, alb, and sng.

Now if i want to reference the Album relation, I can simply use the alias alb (and pray that Intellisense is working).

Notice how in the SELECT, it is immediately clear what fields I’m using and which relations they come from. This is simply because I’ve chosen sensibly named aliases.

I’ve chosen to use short aliases. So my table references are not as verbose. This mainly comes into play in the JOIN predicates.

You don’t have to alias if you don’t want to

Aliasing is not necessary (except in self-joins!). If you want to, you can avoid aliasing altogether and be explicit with your references like so:

SELECT
	AlbumTrack.AlbumID
,	Album.Title AS AlbumTitle
,	AlbumTrack.SongID
,	AlbumTrack.TrackNo
,	Song.Name AS SongTitle
FROM	AlbumTrack
INNER JOIN	Album
ON AlbumTrack.AlbumID = Album.AlbumID
INNER JOIN	music.Song
ON AlbumTrack.SongID = Song.SongID

You may feel that this is more readable.

And certainly there are situations where it is possible for aliases to make things more complicated instead…

Alphabetical Table Aliases = BAD

Let’s take the original query and change all the aliases to letters of the alphabet.

We often find ourselves doing this. It’s quick and easy and requires no planning. But in terms of readability, it has no benefit. And down the road you may be making things harder for yourself.

SELECT
	a.AlbumID
,	b.Title AS AlbumTitle 
,	a.SongID
,	a.TrackNo
,	c.Name AS SongTitle
FROM		AlbumTrack 	AS a
INNER JOIN	Album 		AS b
ON a.AlbumID = b.AlbumID
INNER JOIN	Song 		AS c
ON a.SongID = c.SongID;

Why alphabet?

One idea here is that we can be consistent across all our queries with our table aliases. That way, we always know that the first table is a, followed by b and so on…

Indeed, the alphabet is an ordered sequence and if the ordering of our tables is important then we might want to represent that importance via our aliases.

But aliases are just syntactic sugar. As I mentioned above, you don’t even need to have aliases. The point of an alias is to improve readability.

Aliases are not there to convey the structure of the code. Let the code speak for itself.

Readability

Let’s focus on this part:

SELECT
	a.AlbumID
,	b.Title AS AlbumTitle 
,	a.SongID
,	a.TrackNo
,	c.Name AS SongTitle

Try to discern which tables the fields belong to.

Obviously, a.AlbumID comes from a. Likewise, b.Title comes from b. But what tables are these?

In order for the alias to have any meaning, we need to peruse the query in its entirely. And we will need to do this every time because we won’t remember! Ultimately, these aliases provide no benefit in readability.

Join Order

Furthermore, the ordering of our tables is not important when it comes to the INNER JOIN. And when the

An INNER JOIN is commutative.

FROM		AlbumTrack 	AS trk
INNER JOIN	Album 		AS alb
ON trk.AlbumID = alb.AlbumID

The above could be written as below and produce the same result.

FROM		Album 		AS alb
INNER JOIN	AlbumTrack	AS trk
ON alb.AlbumID = trk.AlbumID

An INNER JOIN is associative. The order of the groupings of multiple joins does not matter:

FROM		AlbumTrack 	AS trk
INNER JOIN	Album 		AS alb
ON trk.AlbumID = alb.AlbumID
INNER JOIN	Song 		AS sng
ON trk.SongID = sng.SongID;

The above is equivalent to:

FROM		AlbumTrack 	AS trk
INNER JOIN	Song 		AS sng
ON trk.SongID = sng.SongID
INNER JOIN	Album 		AS alb
ON trk.AlbumID = alb.AlbumID;

In terms of the execution plan, there may be some optimal ordering. However, the query optimizer will figure that out for you.

Column Aliases

SELECT
	alb.Title AS AlbumTitle
... 

In the above example, we rename alb.Title to AlbumTitle.

Note how it is still obvious that alb.Title is a projection of Album, thanks to the alias.

In Relational Algebra, this is simple a rename.