General rules for simplifying SQL statements

Question

I\'m looking for some \"inference rules\" (similar to set operation rules or logic rules) which I can use to reduce a SQL query in complexity or size. Does there exist some

Answer 1

Here's a few from working with Oracle 8 & 9 (of course, sometimes doing the opposite might make the query simpler or faster):

Parentheses can be removed if they are not used to override operator precedence. A simple example is when all the boolean operators in your where clause are the same: where ((a or b) or c) is equivalent to where a or b or c.

A sub-query can often (if not always) be merged with the main query to simplify it. In my experience, this often improves performance considerably:

select foo.a,
       bar.a
  from foomatic  foo,
       bartastic bar
 where foo.id = bar.id and
       bar.id = (
         select ban.id
           from bantabulous ban
          where ban.bandana = 42
       )
;

is equivalent to

select foo.a,
       bar.a
  from foomatic    foo,
       bartastic   bar,
       bantabulous ban
 where foo.id = bar.id and
       bar.id = ban.id and
       ban.bandana = 42
;

Using ANSI joins separates a lot of "code monkey" logic from the really interesting parts of the where clause: The previous query is equivalent to

select foo.a,
       bar.a
  from foomatic    foo
  join bartastic   bar on bar.id = foo.id
  join bantabulous ban on ban.id = bar.id
 where ban.bandana = 42
;

If you want to check for the existence of a row, don't use count(*), instead use either rownum = 1 or put the query in a where exists clause to fetch only one row instead of all.