Refactoring SQL

Question

Are there any formal techniques for refactoring SQL similar to this list here that is for code?

I am currently working on a massive query for a particular report and I'm sure there's plenty of scope for refactoring here which I'm just stumbling through myself bit by bit.

Answer 1

I have never seen an exhaustive list like the sample you provided.

The most effective way to refactor sql that I have seen is to use the with statement. It allows you to break the sql up into manageable parts, which frequently can be tested independently. In addition it can enable the reuse of query results, sometimes by the use of a system temporary table. It is well worth the effort to examine.

Here is a silly example

WITH 
mnssnInfo AS
(
    SELECT SSN, 
           UPPER(LAST_NAME), 
           UPPER(FIRST_NAME), 
           TAXABLE_INCOME,          
           CHARITABLE_DONATIONS
    FROM IRS_MASTER_FILE
    WHERE STATE = 'MN'                 AND -- limit to Minne-so-tah
          TAXABLE_INCOME > 250000      AND -- is rich 
          CHARITABLE_DONATIONS > 5000      -- might donate too
),
doltishApplicants AS
(
    SELECT SSN, SAT_SCORE, SUBMISSION_DATE
    FROM COLLEGE_ADMISSIONS
    WHERE SAT_SCORE < 100          -- Not as smart as the average moose.
),
todaysAdmissions AS
(
    SELECT doltishApplicants.SSN, 
           TRUNC(SUBMISSION_DATE)  SUBMIT_DATE, 
           LAST_NAME, FIRST_NAME, 
           TAXABLE_INCOME
    FROM mnssnInfo,
         doltishApplicants
    WHERE mnssnInfo.SSN = doltishApplicants.SSN
)
SELECT 'Dear ' || FIRST_NAME || 
       ' your admission to WhatsaMattaU has been accepted.'
FROM todaysAdmissions
WHERE SUBMIT_DATE = TRUNC(SYSDATE)    -- For stuff received today only

One of the other things I like about it, is that this form allows you to separate the filtering from the joining. As a result, you can frequently copy out the subqueries, and execute them stand alone to view the result set associated with them.

Answer 2

There is a book on the subject: "Refactoring Databases". I haven't read it, but it got 4.5/5 stars on Amazon and is co-authored by Scott Ambler, which are both good signs.