SEELCT query

Question

Hi all,
I have a table with the following fields: user_id, opt_col_id, width, ordering, udd_id
user_id is the PK. In this table either opt_col_id is null and udd_id has the value or vice-versa.
If opt_col_id is not null, it will be equal to: ref_opt_col.ID
If udd_id is not null, it will be equal to: customers_udd.udd_id
So I have the id (either opt_col_id or udd_id) in the user_opt_col table and I need to get the equvalent labels from either ref_opt_col or customers_udd tables.
If I run the following queries, I can get the data:
select u.opt_col_id, r.opt_col_label, u.ordering, u.width from ref_opt_col r, user_opt_col u, users users where r.id = u.opt_col_id and u.user_id = users.username and u.user_id = '?'
select u.udd_id, c.udd_label, u.ordering, u.width from user_opt_col u, users users, customers_udd c where u.user_id = users.username and u.udd_id = c.udd_id and u.user_id = '?'
Is there anyway that I can combine these two queries together and run only one SELECT query?
Any help is gretly appreciated.

Answer 1

Well, for an Oracle database, I would do it as:
SELECT NVL(u.opt_col_id, u.udd_id) as id,
       DECODE(u.opt_col_id, null, c.udd_label, r.opt_col_label) as label,
       u.ordering,
       u.width
FROM user_opt_col u,
     ref_opt_col r,
     customers_udd c,
     users users
WHERE u.user_id = users.username
  AND u.opt_col_id = r.id(+)
  AND u.udd_id = c.udd_id(+)
  AND u.user_id = ?DECODE is pretty Oracle-specific but many DBs have ways to do similar inline if-else tests on values. If your DB doesn't, you could return both ids and labels and sort out which one to use in the Java.
The (+) is an outer join; any self-respecting DB supports that, although possibly with different syntax.
Don't assume that doing it as one select is going to be faster (it likely will be on a sucky network or with a decent database product); the above query does quite a bit more work than the two simpler queries.
A whole different way of doing it would be with a stored procedure; a procedure might select the join of user_opt_col and users, inspect the id values, select whichever row it needs to, and return you the right results. Personally, I'd prefer the one query approach over an SP, in the abstract, but it's a judgment call and depends on your DB and your comfort with stored procedures.