SQL优化 ----- 优化 GROUP BY

提高GROUP BY 语句的效率, 可以通过将不需要的记录在GROUP BY 之前过滤掉.下面两个

查询返回相同结果但第二个明显就快了许多.

低效:

SELECT JOB , AVG(SAL)
FROM EMP
GROUP JOB
HAVING JOB = ‘PRESIDENT’
OR JOB = ‘MANAGER’

 

高效:

SELECT JOB , AVG(SAL)
FROM EMP
WHERE JOB = ‘PRESIDENT’
OR JOB = ‘MANAGER’
GROUP JOB