在表值函数中使用CTE的问题

Question

I am trying to avoid using a scalar valued functions in my project, so I decided to try and convert one of them into a table valued function using a CTE.

I understand that the performance of scalar valued functions is poor because it they have to be executed for each row, and SQL server cannot optimise it in any way (ie it acts as a black box).

Here is my first attempt at converting it into a table valued function...

CREATE FUNCTION [dbo].[fn_get_job_average] (@jobnumber VARCHAR(50))
RETURNS TABLE AS RETURN
(   

  WITH JobAverage AS
  (
    SELECT job.Jobnumber, CAST(AVG(CAST(jobMark.Mark AS DECIMAL(18,1))) AS DECIMAL(18,1)) AS Average
    FROM  job 
      INNER JOIN jobMark 
        ON job.Guid = jobMark.Guid
    WHERE job.Jobnumber = @jobnumber
    GROUP BY job.Jobnumber
  )
  SELECT Jobnumber,
    CASE
      WHEN EXISTS(SELECT * FROM JobAverage) THEN Average
      ELSE 0.0 -- This never executes???, i.e. for job records that don't have a mark nothing is returned
    END AS Average
  FROM JobAverage
)

I want to output a table with the job number and average score.

For jobs that do have the mark, it appears to be OK. That is, the average is returned along with the jobnumer.

For jobs that do not have a mark, it seems to go wrong. The ELSE part of the statement does not execute. That is, I don't get 0.0 returned as a the job average. No records are returned. Am I missing something?

Sorry I am not an experienced SQL developer, so I might have a few glaring mistakes in the above code. However, I am confused why a it doesn't work.

Thanks in advance.

Answer 1

Not tested but something like this should do what you want.

   SELECT job.Jobnumber, COALESCE(CAST(AVG(CAST(jobMark.Mark AS DECIMAL(18,1))) AS DECIMAL(18,1)), 0.0) AS Average
    FROM  job 
      LEFT OUTER JOIN jobMark 
        ON job.Guid = jobMark.Guid
    WHERE job.Jobnumber = @jobnumber
    GROUP BY job.Jobnumber

No need to use a CTE.

BTW: What you do is that you check for Exists in the CTE Jobnumber in the case statement. If there are no rows in the CTE you will end up in the else part but since you use the CTE Jobnumber in the from clause of the main query you will not get any rows because the CTE Jobnumber did not return any rows.

So to be perfectly clear of what is happening. The case statement will never be executed if there are no rows in the CTE Jobnumber .

Answer 2

EXISTS(SELECT * FROM JobAverage) means "are there any rows in at all in the whole of JobAverage".

Yes, there are of course because the CASE is executing in the output rows of JobAverage

What you want is this I think:

Average of marks per job.
Zero where no marks for a job

SELECT
    job.Jobnumber,
    ISNULL(
       CAST(AVG(CAST(jobMark.Mark AS DECIMAL(18,1))) AS DECIMAL(18,1))
      ,0) AS Average
FROM 
      job 
      LEFT JOIN
      jobMark ON job.Guid = jobMark.Guid
WHERE job.Jobnumber = @jobnumber
GROUP BY job.Jobnumber