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  1. Put the most frequently occurring conditions first. A CASE statement stops evaluations when the first TRUE test is fulfilled.
  2. When you are counting or summing. Do not use 0 (Zero), instead use NULL. Reason being a 0 (Zero) is added to your count/summation, whereas, a NULL is ignored. Depending on the amount of 0’s being added to your total, this can represent a significant CPU savings.
  3. Example:

, SUM(CASE WHEN ACT_INDA = ‘Y’ THEN 0
WHEN ACT_INDB = ‘Y’ THEN 0
WHEN ACT_INDC = ‘Y’ THEN 1
ELSE 0
END ) AS A
Instead use:
, SUM(CASE WHEN ACT_INDA = ‘Y’ THEN NULL
WHEN ACT_INDB = ‘Y’ THEN NULL
WHEN ACT_INDC = ‘Y’ THEN 1
ELSE NULL
END ) AS A

Important SQL CODES and ABEND CODES
SORT JOIN – TO JOIN TWO FILES BASED ON A KEY
KNOW YOUR MAINFRAME
REXX – INITIAL SETUP
DB2 SQL Query to read COMP (COBOL) data stored in CHAR column
DB2 Performance – Using SET vs SYSDUMMY1 table
DB2 Performace – Predicates Processing Order
DB2 External Stored Procedures
DB2 UPDATE QUERY – UPDATE TABLE1 FROM TABLE2 DATA
DB2 PERFORMANCE ISSUE (When using EXISTS)
HOW DOES DB2 INTERNALLY (PHYSICALLY) STORE THE DATA
HOW DO YOU FIND WHO HAS ACTUALLY LOADED THE DB2 TABLE RECENTLY
TERMINATE A STOPPED DB2 UTILITY
SELECT UNIQUE RECORDS USING “GROUP BY”
DB2 and SQL INTERVIEW QUESTIONS
DB2 TIPS
Optimistic Locking vs. Pessimistic Locking
DB2 BIND OPTIONS and ISOLATION
Is SCHEMA name necessary for DYNAMIC QUERY
DB2 SQL – REPLACE CHARACTERS WITH ACCENTS (NON ENGLISH ALPHABETS)
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