Question

I have three tables AA, BB and CC. BB and CC has one to one mapping to table AA ( with foreign key column : aa_id in both tables). Provided one condition that only either BB or CC can have reference to a row of table AA.

I would like to select latest 1000 rows from AA either joining with BB or CC.
I could do separate queries to select 500 rows joining with BB and CC separately and make 1000. But this will not guarantee that I am getting latest 1000 rows from AA table.

So, I came up with following query to select record from AA by joining with BB and CC at same time. But due to my restriction ( only either BB or CC can have reference to a row of table AA), I am getting 0 rows from query below.

SELECT  * FROM AA 
       INNER JOIN BB  ON (
            AA.id = BB.aa_id
            AND SOME_CONDITION
        ) 
        INNER JOIN CC  ON (
            AA.id = CC.aa_id
            AND SOME_CONDITION
        )
ORDER BY
AA.id DESC limit 1000;

Could you please tell me how to do this? Is there any way so that I can OR these two JOIN conditions?

Answer 1

just change your INNER JOINs to LEFT OUTER JOINs and you’ll be fine. you may want to read this article on codinghorror.com for an explanation of different join-types.

your result should look like this:

SELECT * FROM AA 
  LEFT OUTER JOIN BB  ON (
    AA.id = BB.aa_id
    AND /* SOME_CONDITION */
  ) 
  LEFT OUTER JOIN CC  ON (
    AA.id = CC.aa_id
    AND /* SOME_CONDITION */
  )
ORDER BY AA.id DESC
LIMIT 1000;