Avoid using the HAVING clause, HAVING only filter the result set after the fishes retrieve all records. This process requires a sort, total and so on. If you can use a WHERE clause to limit the number of records that can reduce this overhead .
E.g:
Inefficient:
SELECT REGION，AVG(LOG_SIZE)
FROM LOCATION
GROUP BY REGION
HAVING REGION REGION != ‘SYDNEY’
AND REGION != ‘PERTH’
Efficient
SELECT REGION，AVG(LOG_SIZE)
FROM LOCATION
WHERE REGION REGION != ‘SYDNEY’
AND REGION != ‘PERTH’
GROUP BY REGION
( The HAVING The conditions for some of the more general set of functions , such as COUNT () and the like . Besides , the general conditions should be written in the WHERE clause )