Count Each Teacher’s Distinct Subjects across Departments

### Problem Return teacher_id and cnt, the number of different subject_id values taught by that teacher. Teaching the same subject in several departments counts once. The source key is (subject_id,dept_id), and result row order is unrestricted. ### Tables Table: `Teacher` ```text +-------------+------+ | Column Name | Type | +-------------+------+ | teacher_id | int | | subject_id | int | | dept_id | int | +-------------+------+ (subject_id, dept_id) is the primary key (combinations of columns with unique values) of this table. Each row in this table indicates that the teacher with teacher_id teaches the subject subject_id in the department dept_id. ``` ### Example ```text Teacher (teacher_id,subject_id,dept_id): (1,8,2), (1,8,3), (1,9,2), (2,7,2) Output (teacher_id,cnt): (1,2), (2,1) ``` ### What a Strong Answer Covers - Count each subject once for a teacher, even if the teacher covers it in several departments. - Return the requested count for each teacher in the table. ### Follow-up Questions - How would you report both distinct subjects and distinct departments? - How would you include teachers with no assignments if another table listed all teachers?