The document provides tips for optimizing DB2 SQL queries, highlighting the differences between 'union' and 'union all', and the efficiency of using 'not exists' over 'not in'. It emphasizes the importance of reducing the number of joins and columns in group by statements to improve performance. Additionally, it advises using joins instead of complex programming logic or sub-queries to enhance CPU efficiency.

1. DB2 SQL – PART 1
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2. Union Vs. Union All
• Union - will takes more time. Use it unless it is necessary
• Union All - we can use when to retain duplicates

3. Not Exists vs. Not in
• Not Exists – takes less CPU time
• NOT IN – Takes more CPU time. Before executing SQL it
creates result set

4. Reduce No of Tables in Join
• We can join 255 tables
• Practically it is less
• More joins takes more time to response
• Before JOIN, judge it how many tables really required. 5-7
tables is good

5. Denormalization
• Instead of joins use Denormalization objectively
• Denormalization eliminates, JOINS

6. Reduce No of Rows in Joins
SELECT E.LASTNAME, E.FIRSTNME FROM
DSN8610.DEPT D, DSN8610.EMP E
WHERE D.ADMRDEPT = 'D01'
AND D.DEPTNO = E.WORKDEPT
AND E.SEX = 'M'
AND E.SALARY >= 40000.00;
More qualifications takes more CPU time

7. Use Joins instead to write logic
• Always use JOINS in DB2 programs, instead of writing complex
logic
• Avoid using programming logic, unless it is necessary

8. Use JOINS instead of Sub-queries
• SELECT EMPNO, LASTNAME FROM DSN8610.EMP,
DSN8610.DEPT WHERE WORKDEPT = DEPTNO AND
DEPTNAME = 'PLANNING';
is usually more efficient than the subquery:
• SELECT EMPNO, LASTNAME FROM DSN8610.EMP
WHERE WORKDEPT IN (SELECT DEPTNO FROM
DSN8610.DEPT WHERE DEPTNAME = 'PLANNING');

9. Limit the Columns in Group By
• Always Reduce no of columns in Group by.
• It avoids more Sorts, and less CPU time.

10. Thank You
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