如何在不破坏 SQL 逻辑的情况下将 JOINS 转换为子查询

Question

我在 T-SQL 中使用多个联接时遇到了性能问题，如果有人可以帮助我将这些联接转换为子查询，那就太好了。

每当我尝试将连接更改为子查询时，我都会丢失特定表的名称声明。例如，如果我尝试将专辑连接（这是下面代码的第一个连接）转换为子查询，我将失去别名“AS a”并且“a.Title AS Album”停止工作，所以我不知道这是怎么回事会完成的。如果有人给我一个例子，它应该如何适用于其中一种情况，我想我将能够重建所有这些情况。

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

在子查询中加入连接可能不是最好的解决方案

假设您已经为每个被替换的表建立了外键索引

table  Artist index on column (ArtistId)
table  PlaylistTrack index on column (TrackId)
table  Playlist index on column (PlaylistId)
table  MediaType index  on column ( MediaTypeId )
.....

为了性能，请确保您有索引

table  track a composite index  on column  (AlbumId, TrackId, MediaTypeId )
table  Album  a cmposite index  on column ( AlbumId, ArtistId )

Answer 2

你的意思是什么类型的子类？比如：

SELECT t.TrackId, 
    t.[Name] AS Track, 
    (SELECT title FROM dbo.Album WHERE AlbumId = t.AlbumId) AS AlbumTitle

如果您删除对Album 的加入，则不会发生这种情况，因为您需要专辑引用才能找到艺术家。如果你想加入一个子查询，你可以这样做，并保留别名：

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

但从逻辑上讲，这和你现在的完全一样，查询优化器生成的计划不会有任何差异。即使这样：

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

将产生完全相同的计划。我们可能已经将Album Artist 连接移动到连接子查询，但本质上，它仍然是相同的东西——内部连接。

这样的事情不会对您的表现有所帮助。可能对您有所帮助的是在这些表上创建索引。如果您经常执行这种查询，您还可以创建索引视图，例如：

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);

这会减慢对这些表的插入速度，但在 BoughtTracks 上选择会很快（您也可以在该视图上创建其他索引），例如：

SELECT * 
  FROM BoughtTracks WITH (NOEXPAND) -- NOEXPAND is important
 WHERE CustomerName = 'Joe Smith'

执行速度可能比您当前的查询快几个数量级，这当然取决于您的数据大小。特别是如果您在其上创建索引

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