如何在嵌套的 SQL SELECT 查询中链接两个值？

Question

I have an SQL Server table JobFiles with columns JobId and FileId. This table maps which file belongs to which job. Each job can "contain" one or more files and each file can be "contained" in one or more job. For every pair such that job M contains file N there's a row (M,N) in the table.

I start with a job id and I need to get a list of all files such that they belong to that job only. I have hard time writing a request for that. So far I've crafted the following (pseudocode):

SELECT FileId FROM JobFiles WHERE JobId=JobIdICareAbout AND NOT EXISTS
    (SELECT * FROM JobFiles WHERE FileId=ThatSameFileId AND JobId<>JobIdICareAbout);

The above I believe would work but I have a problem of how to map ThatSameFileId onto the FileId returned from the outer SELECT so that the database knows that those are the same.

How do I do that? How do I tell the database that FileId in the outer SELECT must be equal to the FileId in the inner SELECT ?

Answer 1

How about using NOT IN :

SELECT FileId FROM JobFiles WHERE JobId=JobIdICareAbout AND FileID NOT IN
    (SELECT FileId FROM JobFiles WHERE JobId<>JobIdICareAbout);

And a slight variation on that:

SELECT FileId FROM JobFiles WHERE JobId=JobIdICareAbout
EXCEPT
SELECT FileId FROM JobFiles WHERE JobId<>JobIdICareAbout

Answer 2

I'm taking another approach here but if I understood your problem correctly, it would generate the result you need.

Gist of it goes like this

• Get all FileId's from the JobId you care about
• JOIN back with the JobFiles tabel
• Retain only those FileId's that have no other JobId using a HAVING clause.

SQL Statement

SELECT  jf1.FileId
FROM    JobFiles jf1
        INNER JOIN (
          SELECT FileId
          FROM   JobFiles
          WHERE  JobID = JobIdICareAbout
        ) jf2 ON jf2.FileID = jf1.FileID
GROUP BY
        jf1.FileId
HAVING COUNT(*) = 1

Answer 3

Another approach:
LEFT JOIN will only find rows if this file is linked to another job, checking for IS NULL removes those files:

SELECT JobFiles.FileId
FROM JobFiles
LEFT JOIN JobFiles OtherJobFiles ON (     OtherJobFiles.FileId = JobFiles.FileId
                                      AND OtherJobFiles.JobId <> JobIdICareAbout )
WHERE JobFiles.JobId=JobIdICareAbout
  AND OtherJobFiles.FileId IS NULL

Answer 4

It seems you have a lot of (different but) working answers/queries. Here's one more using NOT EXISTS . It's only a correction of what you tried:

SELECT jf.FileId
FROM JobFiles jf 
WHERE jf.JobId = JobIdICareAbout
  AND NOT EXISTS
    ( SELECT 1
      FROM JobFiles jf2
      WHERE jf2.FileId = jf.FileId 
        AND jf2.JobId <> JobIdICareAbout
    )