leetcode-2356. 每位教师所教授的科目种类的数量

题目：

解答过程：

一开始输入下面这个查询语句

select teacher_id,count(subject_id) as cnt
from Teacher
group by teacher_id;

 输出结果有问题：

 因为1号教师在3、4系都有教2科目，所以有重复的数据。

在count()中加入distance()去重就可以了

select teacher_id,count(distinct(subject_id)) as cnt
from Teacher
group by teacher_id;