How to remove unnecessary join, update Where conditions

Question

I've got three tables in a MySql database that I'm joining in a query to get id/value pairs.

   |    A     |       B      |     C         |
   | -------- |--------------|---------------|
   |   id     |  id          |  id           |
   |   name   |  fooId       |  attributeId  |
   |   desc   |  value       |  displayIndex |
   |   ...    |  attributeId |  ...          |

What I have now is:

SELECT C.id, B.value
  FROM  A, B, C
  WHERE A.id = B.attributeId
    AND A.id = C.attributeId
    AND B.fooId = 25
  ORDER BY C.displayIndex

So basically we're joining B and C through A. It used to be that an entry in the C table had to have a corresponding (parent) entry in the A table. However, that will no longer be the case. The C table will still be MOSTLY controlled by the A table, however, there are some instances when we need a stand alone (always on) entry in the C table.

EDIT

I want all the records from B and C that match on attributeId but I also want any record where C.attributeId = -1. Can someone help with what I'd need to do with this query?

Edit #2

Based on feedback and suggestions you guys have made and some googling I now have this:

(SELECT C.id, B.value, C.displayIndex
  FROM  B, C
  WHERE B.attributeId = C.attributeId
    AND B.fooId = 25)

UNION

(SELECT C.id, null, C.displayIndex
  FROM C
  WHERE C.attributeId = -1)

ORDER BY 3

Is there a better what to do this? Are there any problems with UNION?

Answer 1

I've updated my answer to address the edits from the OP.

This will return all records where the attributeId for tables B and C match,
with B.fooId = 25, OR C.attributeId = -1.
When C.attributeId = -1 and there is no match in table B, NULL will be returned in place of B.Value, which appears to be acceptable based on the Edit #2 from the OP

SELECT C.Id, B.Value, C.displayIndex
FROM C 
LEFT JOIN B ON C.attributeId = B.attributeId
WHERE B.fooId = 25
   OR C.attributeId = -1
ORDER BY C.DisplayIndex ASC