Question

This is a snippet from my stored Proc

SELECT  NULL AS StoryID
      , AlbumID
      , CAST(NULL as varchar) AS StoryTitle
      , AlbumName
      , (SELECT URL FROM AlbumPictures AS AlbumPictures_3 WHERE (AlbumID = Albums.AlbumID) AND (AlbumCover = 'True')) AS AlbumCover
      , Votes
      , CAST(NULL as Int) AS PictureId
      , 'albums' AS tableName
      , (SELECT NestedAlbums.AlbumID FROM NestedAlbums WHERE (AlbumID = Albums.AlbumID)) AS Flag
INTO #Results2
FROM Albums WHERE AlbumID IN (SELECT StringVal FROM funcListToTableInt(@whereAlbumID))

I have used nested selects in my Query above. I am curious to know whether Nested Selects are better than LEFT/Right JOINS OR should i use JOINS?

Table Albums:

Table NestedAlbums:

Answer 1

In general writing this an explicit OUTER JOIN would be better.

SQL Server will probably need to add an Assert to the plan with the sub query version that verifies the sub query only returns at most one row (unless this is guaranteed by a unique index). This can limit the possible transformations available. See Scalar Subqueries for more about this.

Also (though not relevant to the example in your question as both sub queries are different) writing as an explicit JOIN allows you to use multiple columns from the joined table with one lookup whereas using separate similar subqueries would not (SQL Server has no logic to detect the common sub expressions).

Edit:

Following discussion in comments something like

SELECT NULL                      AS StoryID,
       A.AlbumID,
       CAST(NULL AS VARCHAR(30)) AS StoryTitle,
       A.AlbumName,
       AP.URL                    AS AlbumCover,
       A.Votes,
       CAST(NULL AS INT)         AS PictureId,
       'albums'                  AS tableName,
       CASE
         WHEN EXISTS (SELECT *
                      FROM   NestedAlbums NA
                      WHERE  NA.AlbumID = A.AlbumID
                             AND ( AccountId = @AccountId )) THEN 1
         ELSE 0
       END                       AS Flag
INTO   #Results2
FROM   Albums A
       LEFT OUTER JOIN AlbumPictures AP
         ON ( AP.AlbumID = A.AlbumID )
            AND ( AP.AlbumCover = 'True' )
WHERE  A.AlbumID IN (SELECT StringVal
                     FROM   funcListToTableInt(@whereAlbumID)) 

You may notice this still has a sub query in the SELECT list but CASE ... EXISTS will be implemented efficiently as a semi join.

At the moment your query assumes that at most one matching row per album will be returned from AlbumPictures and would error out if this assumption is not true. This changes the semantics in that no error will be returned and you would get multiple rows with the various URLs. If you didn’t want that to happen you would need to define which URL to use and add a GROUP BY