596. Classes More Than 5 Students 【having】

题目

There is a table courses with columns: student and class

Please list out all classes which have more than or equal to 5 students.

For example, the table:

+---------+------------+
| student | class      |
+---------+------------+
| A       | Math       |
| B       | English    |
| C       | Math       |
| D       | Biology    |
| E       | Math       |
| F       | Computer   |
| G       | Math       |
| H       | Math       |
| I       | Math       |
+---------+------------+

Should output:

+---------+
| class   |
+---------+
| Math    |
+---------+

 答案：

select a.class from

(select class, count(distinct student) as num from courses group by class) as a where a.num >=5;



select class from courses group by class having count(distinct student) >= 5;

　　

having与where的区别是having可以使用聚合函数，比如sum，count等