Oracle: Is it possible to choose which table to join based on a column value?

Question

First of all, this isn't my design, and I don't have the option of changing it. I've got three tables that define a relationship between action items and incidents and audits. The tables, in very simplified form, are set up like this:

ACTION_ITEMS
-------
ID    SOURCE_ID    SOURCE_TYPE
1     12345        INC
2     67890        AUD

INCIDENTS
-------
ID
12345

AUDITS
-------
ID
67890

The SOURCE_TYPE column indicates which of the other two tables is relevant to that row. It's simple enough to get the details for all the records by creating separate queries for the incident and audit action items, joining those to the appropriate tables, and then performing a union.

What I'd like to know is if it's possible to conditionally join to a table based on the value of a column. In other words, something like this, but that actually works:

SELECT
   AI.*
  ,INC.*
  ,AUD.*
FROM ACTION_ITEMS AI
JOIN
  CASE AI.SOURCE_TYPE
    WHEN 'INC' THEN INCIDENTS INC ON (AI.SOURCE_ID = INCIDENTS.ID)
    WHEN 'AUD' THEN AUDITS AUD ON (AI.SOURCE_ID = AUDITS.ID)
  END;

**Edited to clarify that I'm seeking to retrieve data from all tables.

Answer 1

No, but you can join both tables and use the condition in your join clause:

SELECT AI.*
FROM ACTION_ITEMS AI
LEFT JOIN INCIDENTS ON AI.SOURCE_TYPE = 'INC' AND (AI.SOURCE_ID = INCIDENTS.ID)
LEFT JOIN AUDITS    ON AI.SOURCE_TYPE = 'AUD' AND (AI.SOURCE_ID = INCIDENTS.ID)

Note that you will be able to access columns from either table depending on the SOURCE_TYPE - if there's no match in the right-hand table those values will be NULL

Answer 2

You appear to be using the tables for filtering purposes, because you are not selecting from them.

Another way to write this is by using conditions in the where clause:

select AI.*
from ACTION_ITEMS AI
where (AI.SOURCE_TYPE = 'INC' and
       exists (select 1 from INCIDENTS where AI.SOURCE_ID = INCIDENTS.ID)
      ) or
      (AI.SOURCE_TYPE = 'AUD' and
       exists (select 1 from AUDITS where AI.SOURCE_ID = AUDITS.ID)
      );

If you want to do this with join s, you have to use left outer join and then explicitly filter for matches:

SELECT AI.*
FROM ACTION_ITEMS AI LEFT JOIN
     INCIDENTS
     ON AI.SOURCE_ID = INCIDENTS.ID LEFT JOIN
     AUDITS
     ON AI.SOURCE_TYPE = 'AUD' AND (AI.SOURCE_ID = AUDITS.ID)
WHERE (AI.SOURCE_TYPE = 'INC' and INCIDENTS.ID is not null) OR
      (AI.SOURCE_TYPE = 'AUD' and AUDITS.ID is not null);