Question

I have an order system. Users with can be attached to different orders as a type of different user. They can download documents associated with an order. Documents are only given to certain types of users on the order. I’m having trouble writing the query to check a user’s permission to view a document and select the info about the document.

I have the following tables and (applicable) fields:

Docs: DocNo, FileNo
DocAccess: DocNo, UserTypeWithAccess
FileUsers: FileNo, UserType, UserNo 

I have the following query:

SELECT Docs.* 
FROM Docs
WHERE DocNo = 1000
  AND EXISTS (
         SELECT * FROM DocAccess
         LEFT JOIN FileUsers
           ON FileUsers.UserType = DocAccess.UserTypeWithAccess 
           AND FileUsers.FileNo = Docs.FileNo /* Errors here */
         WHERE DocAccess.UserNo = 2000 )

The trouble is that in the Exists Select, it does not recognize Docs (at Docs.FileNo) as a valid table. If I move the second on argument to the where clause it works, but I would rather limit the initial join rather than filter them out after the fact.

I can get around this a couple ways, but this seems like it would be best. Anything I’m missing here? Or is it simply not allowed?

Answer 1

I think this is a limitation of your database engine. In most databases, docs would be in scope for the entire subquery — including both the where and in clauses.

However, you do not need to worry about where you put the particular clause. SQL is a descriptive language, not a procedural language. The purpose of SQL is to describe the output. The SQL engine, parser, and compiler should be choosing the most optimal execution path. Not always true. But, move the condition to the where clause and don’t worry about it.