How to turn JOINS into Subqueries without breaking the SQL logic - sql

Question

I have performance issues while using multiple joins in T-SQL and if anyone can help me to turn these joins into sub-queries would be nice.
Whenever i try to change the join into subquery, i'm loosing the name declaration for the specific table. For instance if i try to turn the Album join (which is the first join on code below) into subquery i'm loosing the alias "AS a" and the "a.Title AS Album" stopped working so i have no idea how this would be done. If anyone gives me an example how should it work for one of the cases i suppose i will be able to reconstruct all of them.
SQL
SELECT
t.TrackId,
t.[Name] AS Track,
a.Title AS Album,
aa.[Name] AS Artist,
p.[Name] AS Playlist,
m.[Name] AS MediaType,
il.UnitPrice AS InvoicePrice,
CONCAT(c.FirstName, ' ', c.LastName) AS CustomerName,
CONCAT(e.FirstName, ' ', e.LastName) AS ResponsibleEmployeeName
FROM dbo.Track AS t
INNER JOIN dbo.Album AS a
ON t.AlbumId = a.AlbumId
INNER JOIN dbo.Artist AS aa
ON a.ArtistId = aa.ArtistId
INNER JOIN dbo.PlaylistTrack AS plt
ON t.TrackId = plt.TrackId
INNER JOIN dbo.Playlist AS p
ON p.PlaylistId = plt.PlaylistId
INNER JOIN dbo.MediaType AS m
ON t.MediaTypeId = m.MediaTypeId
INNER JOIN dbo.InvoiceLine AS il
ON t.TrackId = il.TrackId
INNER JOIN dbo.Invoice AS i
ON il.InvoiceId = i.InvoiceId
INNER JOIN dbo.Customer AS c
ON i.CustomerId = c.CustomerId
INNER JOIN dbo.Employee AS e
ON c.SupportRepId = e.EmployeeId
WHERE m.[Name] LIKE '%audio%'
ORDER BY t.[Name] ASC

Answer 1

Turn a join in subquery could be not the best solution
assuming you have already the index for foreign key for each retaled table
table Artist index on column (ArtistId)
table PlaylistTrack index on column (TrackId)
table Playlist index on column (PlaylistId)
table MediaType index on column ( MediaTypeId )
.....
for performance be sure you have index on
table track a composite index on column (AlbumId, TrackId, MediaTypeId )
table Album a cmposite index on column ( AlbumId, ArtistId )

Answer 2

What kind of a subqyery you mean? Something like:
SELECT t.TrackId,
t.[Name] AS Track,
(SELECT title FROM dbo.Album WHERE AlbumId = t.AlbumId) AS AlbumTitle
That's not going if you remove the join to Album, because you need the album reference to get to artist. If you want to join a subquery, you can do that, and keep the alias:
SELECT
t.TrackId,
t.[Name] AS Track,
a.Title AS Album,
aa.[Name] AS Artist,
p.[Name] AS Playlist,
m.[Name] AS MediaType,
il.UnitPrice AS InvoicePrice,
CONCAT(c.FirstName, ' ', c.LastName) AS CustomerName,
CONCAT(e.FirstName, ' ', e.LastName) AS ResponsibleEmployeeName
FROM dbo.Track AS t
INNER JOIN (SELECT * FROM dbo.Album) AS a
ON t.AlbumId = a.AlbumId
-- rest of joins
But logically, this is exactly the same as you have right now, and there will be no difference in the plan generated by the query optimizer. Even this:
SELECT
t.TrackId,
t.[Name] AS Track,
aa.Title AS Album, -- note change here
aa.[Name] AS Artist,
p.[Name] AS Playlist,
m.[Name] AS MediaType,
il.UnitPrice AS InvoicePrice,
CONCAT(c.FirstName, ' ', c.LastName) AS CustomerName,
CONCAT(e.FirstName, ' ', e.LastName) AS ResponsibleEmployeeName
FROM dbo.Track AS t
INNER JOIN (SELECT alb.Title
, art.Name
, alb.AlbumId
FROM dbo.Album alb
INNER JOIN dbo.Artist art
ON art.ArtistId = alb.ArtistID) AS aa
ON t.AlbumId = aa.AlbumId
INNER JOIN dbo.PlaylistTrack AS plt
ON t.TrackId = plt.TrackId
-- rest of them
Will produce the exact same plan. We might have moved Album <-> Artist join to a joined subquery, but essentially, it's still the same thing - inner joins.
Nothing like that will help with your performance. What might help you is to create indexes on these tables. If this is the kind of query you execute a lot, you can also create an indexed view, something like:
CREATE VIEW BoughtTracks
WITH SCHEMABINDING
AS
SELECT
il.InvoiceLineId, -- I'm guessing here, we need a unique ID
t.TrackId,
t.[Name] AS Track,
a.Title AS Album,
aa.[Name] AS Artist,
p.[Name] AS Playlist,
m.[Name] AS MediaType,
il.UnitPrice AS InvoicePrice,
CONCAT(c.FirstName, ' ', c.LastName) AS CustomerName,
CONCAT(e.FirstName, ' ', e.LastName) AS ResponsibleEmployeeName
FROM dbo.Track AS t
INNER JOIN dbo.Album AS a
ON t.AlbumId = a.AlbumId
INNER JOIN dbo.Artist AS aa
ON a.ArtistId = aa.ArtistId
INNER JOIN dbo.PlaylistTrack AS plt
ON t.TrackId = plt.TrackId
INNER JOIN dbo.Playlist AS p
ON p.PlaylistId = plt.PlaylistId
INNER JOIN dbo.MediaType AS m
ON t.MediaTypeId = m.MediaTypeId
INNER JOIN dbo.InvoiceLine AS il
ON t.TrackId = il.TrackId
INNER JOIN dbo.Invoice AS i
ON il.InvoiceId = i.InvoiceId
INNER JOIN dbo.Customer AS c
ON i.CustomerId = c.CustomerId
INNER JOIN dbo.Employee AS e
ON c.SupportRepId = e.EmployeeId
WHERE m.[Name] LIKE '%audio%'
CREATE UNIQUE CLUSTERED INDEX ux ON BoughtTracks (InvoiceLineId);
This will slow down inserts to these tables, but selecting on BoughtTracks will be fast (you can also create additional indexes on that view), something like:
SELECT *
FROM BoughtTracks WITH (NOEXPAND) -- NOEXPAND is important
WHERE CustomerName = 'Joe Smith'
Could execute orders of magnitude faster than your current query, depending of course on the size of your data. Especially if you create an index on it
CREATE INDEX ix_CustomerName ON BoughtTracks (CustomerName)
INCLUDE (...) -- maybe include some columns you know you will need when querying for CustomerName
WHERE (...) -- maybe there are alsways accompanying predicates when querying for CustomerName