Select from different tables based on value

Question

Hi Forum,
I'm stuggling with a problem:
In a table, I have two columns, pref_type and pref_value. Column pref_type stores a number value that is linked to a preference type, whereas column pref_value stores the value of that preference.
Unfortunately, the data model is such that per preference type a different tables needs to be used, so for example, if
pref_type = 1, table to query is A, whereas if
pref_type = 2, table to query is B.
I now want to write a query that resolves the preferences, ie it should branch to the respective table based on pref_type and search for pref_value and deliver back a different column of that respective table.
My first approach was like that:
select decode (pref_value, 1, (select result from A where pref_value = ???),
2, (select result from B where pref_value = ???)
where the three ??? depict the problem I'm having, as Oracle doesn't allow me to have a harmonized subquery here.
How do I achieve this?
Thanks for any help,
J. Sieben

Answer 1

I believe this is what you are trying to get at:
SELECT DECODE(m.pref_type, 1, (SELECT result FROM a
                               WHERE a.pref_value = m.pref_value),
                           2, (SELECT result FROM b
                               WHERE b.pref_value = m.pref_value)
FROM my_table mAlthough, if the tables are large, that might be pretty slow. Better than an outer join, but one of those correlated queries is going to be run for each row in my_table. You could lso try something like:
SELECT m.pref_type, r.result
FROM my_table m,
     (SELECT 'A' tbl, pref_value, result
      FROM a
      UNION ALL
      SELECT 'B', pref_value, result
      FROM b) r
WHERE DECODE(m.pref_type, 1, 'A', 'B') = r.tbl and
      m.pref_value = r.pref_valueHTH
John