What generic techniques can be applied to optimize SQL queries?

Question

What techniques can be applied effectively to improve the performance of SQL queries? Are there any general rules that apply?

Answer 1

Use a with statment to handle query filtering. Limit each subquery to the minimum number of rows possible. then join the subqueries.

WITH
master AS
(
    SELECT SSN, FIRST_NAME, LAST_NAME
    FROM MASTER_SSN
    WHERE STATE = 'PA' AND
          GENDER = 'M'
),
taxReturns AS
(
    SELECT SSN, RETURN_ID, GROSS_PAY
    FROM MASTER_RETURNS
    WHERE YEAR < 2003 AND
          YEAR > 2000
)
SELECT *
FROM master,
     taxReturns
WHERE master.ssn = taxReturns.ssn

A subqueries within a with statement may end up as being the same as inline views, or automatically generated temp tables. I find in the work I do, retail data, that about 70-80% of the time, there is a performance benefit.

100% of the time, there is a maintenance benefit.