SQL selecting rows where one column's value is common across another criteria column

Question

I have a cross reference table that looks like this:

id  document_id  subject_id
1   8            21
2   5            17
3   5            76
4   7                    


        

Answer 1

That's a very interesting question.

I'm assuming you would like a more generalized query, but this is what I would do in the case where you always have the same number of subjects (say two):

 SELECT T.id, T.document_id, T.subject_id
   FROM table T
        INNER JOIN table T1 ON T.document_id = T1.document_id AND T1.subject_ID = 17
        INNER JOIN table T2 ON T.document_id = T2.document_id AND T2.subject_ID = 76            

Of course, you could add yet another INNER JOIN to add another subject ID.. But I admit it's not a very good general solution.