sql语句中 having与where区别

HAVING语句通常与GROUP BY语句联合使用，用来过滤由GROUP BY语句返回的记录集。

HAVING语句的存在弥补了WHERE关键字不能与聚合函数联合使用的不足。

CREATE TABLE `tp5_student` (

  `id` int(11) DEFAULT NULL,

  `no` int(11) DEFAULT NULL COMMENT '学号',

  `course` varchar(255) DEFAULT NULL COMMENT '课程',

  `score` int(11) DEFAULT NULL COMMENT '分数'

) ENGINE=InnoDB DEFAULT CHARSET=utf8

SELECT id,no, COUNT(course) as numcourse, AVG(score) as avgscore

FROM tp5_student

GROUP BY no

SELECT id,no, COUNT(course) as numcourse, AVG(score) as avgscore

FROM tp5_student

GROUP BY no HAVING  AVG(score)>=85

SELECT id,no, COUNT(course) as numcourse, AVG(score) as avgscore

FROM tp5_student

WHERE  id>3

GROUP BY no