In SQL statements with sub-queries, paying particular attention to reducing the query to the table.
E.g:
Inefficient
SELECT TAB_NAME
FROM TABLES
WHERE TAB_NAME = ( SELECT TAB_NAME
FROM TAB_COLUMNS
WHERE VERSION = 604)
AND DB_VER= ( SELECT DB_VER
FROM TAB_COLUMNS
WHERE VERSION = 604)
Efficient
SELECT TAB_NAME
FROM TABLES
WHERE (TAB_NAME,DB_VER)
= ( SELECT TAB_NAME,DB_VER)
FROM TAB_COLUMNS
WHERE VERSION = 604)
Update Column plurality example:
Inefficient:
UPDATE EMP
SET EMP_CAT = (SELECT MAX(CATEGORY) FROM EMP_CATEGORIES),
SAL_RANGE = (SELECT MAX(SAL_RANGE) FROM EMP_CATEGORIES)
WHERE EMP_DEPT = 0020;
Efficient:
UPDATE EMP
SET (EMP_CAT, SAL_RANGE)
= (SELECT MAX(CATEGORY) , MAX(SAL_RANGE)
FROM EMP_CATEGORIES)
WHERE EMP_DEPT = 0020;