596. Classes More Than 5 Students

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:

For example:

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

Should output:

+———+
| class |
+———+
| Math |
+———+
Note:
The students should not be counted duplicate in each course.

解答：输出超过5个学生班级即可，没注意Note里说不可重复计算学生数量，用例里面也确实出现了重复项，加上DISTINCT就可以了：

  • 一般方法：
SELECT class FROM courses GROUP BY class HAVING COUNT(DISTINCT student) >= 5;
  • 子查询
SELECT class FROM (SELECT COUNT(DISTINCT student) AS num,class FROM courses GROUP BY class) AS T WHERE T.num>=5;