group by

group by: 根据by对数据按照哪个字段进行分组，或者是哪几个字段进行分组

select   字段    from   表名   group  by    字段    having    过滤条件

eg：

SELECT id, COUNT(course) as numcourse, AVG(score) as avgscore FROM student GROUP BY id HAVING AVG(score)>=80;

SELECT name FROM student GROUP BY name HAVING min(score)>=80;