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

I like to replace all sort of subselect by join query.

This one is obvious :

SELECT  *
FROM    mytable mo
WHERE   EXISTS
        (
          SELECT  *
          FROM    othertable o
          WHERE   o.othercol = mo.col
        )

by

SELECT  mo.*
FROM    mytable mo inner join othertable o on o.othercol = mo.col

And this one is under estimate :

SELECT  *
FROM    mytable mo
WHERE   NOT EXISTS
        (
          SELECT  *
          FROM    othertable o
          WHERE   o.othercol = mo.col
        )

by

SELECT  mo.*
FROM    mytable mo left outer join othertable o on o.othercol = mo.col
WHERE   o.othercol is null

It could help the DBMS to choose the good execution plan in a big request.