Question

This one really has me stumped. I have a documents table which hold info about the documents, and a procedures table, which is kind of like a revisions table for each document. What I need to do is write a select statement which gives me all of the documents where all of the procedures have the status “work_in_progress”. Here’s an example

procedures table:

document_id | status
1           | 'wip' 
1           | 'wip'
1           | 'wip'
1           | 'approved'
2           | 'wip'
2           | 'wip'
2           | 'wip'

Here, I would want my query to only return document id 2, because all of its statuses are work_in_progress. I DO NOT want document_id 1 since one of its statuses is ‘approved’. I believe this is relational division I want, but I’m not sure where to start. This is MySQL 5.0 FYI.

Answer 1

I would do this as:

SELECT document_id, document_name, ...
FROM documents AS d
WHERE NOT EXISTS (SELECT document_id
                  FROM procedures
                  WHERE document_id = d.document_id
                      AND status != 'wip');

That is, “show me all documents where there aren’t any rows in procedures with a status other than ‘wip'”.

So it’s coming at it a little sideways from the way you described it. Instead of “all the rows are ‘wip'”, you verify that there aren’t any rows that are not ‘wip’.