Sql-server – CTE To Use Alternate Select When Select Returns No Rows

cteselectsql server

I have created an involved CTE, with no recursion, but with multiple selects being done as it processes the business logic of the requirement. Though, I have been advised that the logic needed for the scenario where no rows are returned, should instead to return all rows found from one of the middle CTEs select instead of the final operation.

How can this be achieved?

Note the following example is basic interpretation of my process and the operation in question occurs after multiple cascaded CTE selects are done an essentially chained together.

Example (as found on SQLFiddle)

In a User Table there are two records where the ids of UserIds are 1 and 2. The first CTEs select, TargetUsers will return no rows requiring the the third CTE, ComputeWithUsers, to use the AllUsers instead. 

WITH TargetUsers AS
(
  SELECT * 
  FROM Users
  WHERE UserId > 5
)
, AllUsers AS
(
   SELECT * FROM Users
)
, ComputeWithUsers
(
-- This CTE will determine it needs to use `AllUsers` and not `TargetUsers`
)

Or is there a way to make the TargetUsers select smart enough to extract all users when its main operation returns no rows?

Best Answer

Here is the way I would do this, assuming the "do it all in one query" isn't just a pointless, artificial requirement.

SELECT cols INTO #x FROM dbo.Users WHERE UserID > 5;

IF @@ROWCOUNT > 0
  SELECT cols FROM #x;
ELSE
  SELECT cols FROM dbo.Users;

If the "do it all in one query" is a hard requirement and you can't use a #temp table, or you really have complex logic you've dumbed down from us and/or don't want to repeat, then you're going to have to pay for those choices in performance, I'm afraid. This looks like it should be quite efficient, but there are going to be more hits to the base table than most people would hope / expect:

;WITH TargetUsers AS 
(
  SELECT cols FROM dbo.Users WHERE UserID > 5
),
AllUsers AS 
(
  SELECT cols FROM dbo.Users
  WHERE NOT EXISTS (SELECT 1 FROM TargetUsers)
    -- this will only return rows if the first CTE is empty
),
ComputeWithUsers AS
(
  SELECT cols FROM TargetUsers 
  UNION ALL 
  SELECT cols FROM AllUsers
)
...;

In fact if UserID is the primary key and the filter really is on UserID, this might be better as follows (but you'll have to check the plans to see, depends on a variety of factors):

;WITH TargetUserIDs AS 
(
  SELECT UserID FROM dbo.Users WHERE UserID > 5
),
AllUserIDs AS 
(
  SELECT UserID FROM dbo.Users
  WHERE NOT EXISTS (SELECT 1 FROM TargetUsers)
    -- this will only return rows if the first CTE is empty
),
ComputeWithUsers AS
(
  SELECT UserID FROM TargetUserIDs 
  UNION ALL 
  SELECT UserID FROM AllUserIDs
)
...
-- and join to dbo.Users to get other columns necessary
-- deferring that to as late as possible

Hopefully your queries are using suboptimal practices due to simplification, and not like that in production, but please give these a read:

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