Number of Unique Subjects Taught by Each Teacher

Task: Write a solution to calculate the number of unique subjects each teacher teaches in the university.

Table: Teacher

+-------------+------+
| Column Name | Type |
+-------------+------+
| teacher_id  | INT  |
| subject_id  | INT  |
| dept_id     | INT  |
+-------------+------+

Input: Teacher table:

+------------+------------+---------+
| teacher_id | subject_id | dept_id |
+------------+------------+---------+
| 1          | 2          | 3       |
| 1          | 2          | 4       |
| 1          | 3          | 3       |
| 2          | 1          | 1       |
| 2          | 2          | 1       |
| 2          | 3          | 1       |
| 2          | 4          | 1       |
+------------+------------+---------+

Output:

+------------+-----+
| teacher_id | cnt |
+------------+-----+
| 1          | 2   |
| 2          | 4   |
+------------+-----+