避免使用HAVING子句, HAVING 只会在检索出所有记录之后才对结果集进行过滤. 这个处理需要排序,总计等操作. 如果能通过WHERE子句限制记录的数目,那就能减少这方面的开销.
例如:
低效:
SELECT REGION,AVG(LOG_SIZE)
FROM LOCATION
GROUP BY REGION
HAVING REGION REGION != ‘SYDNEY’
AND REGION != ‘PERTH’
高效
SELECT REGION,AVG(LOG_SIZE)
FROM LOCATION
WHERE REGION REGION != ‘SYDNEY’
AND REGION != ‘PERTH’
GROUP BY REGION
( HAVING 中的条件一般用于对一些集合函数的比较,如COUNT() 等等. 除此而外,一般的条件应该写在WHERE子句中)