merging two SELECT queries

Question

I am either getting old or the queries that I need to write are getting more and more complicated. The following query will get all the tasks associated with the us

Answer 1

You can do this in a single query:

SELECT t.date
  FROM TASKS t
 WHERE t.user_id = 1
    OR EXISTS(SELECT NULL
                FROM URLS u
               WHERE u.id = t.url_id
                 AND u.user_id = 1)

However, OR is a notoriously bad performer -- it splinters the execution plan. Splitting the query, joining the result sets can be done using the UNION or UNION ALL operators. UNION removes duplicates from the final result set; UNION ALL does not remove duplicates and is faster for it.

SELECT t.date
  FROM TASKS t
 WHERE t.user_id = 1
UNION ALL
SELECT t.date
  FROM TASKS t
 WHERE EXISTS(SELECT NULL
                FROM URLS u
               WHERE u.id = t.url_id
                 AND u.user_id = 1)

Know your data, so you know which UNION operator best serves your needs.