2356. Number of Unique Subjects Taught by Each Teacher.sql

-- Problem
-- +-------------+------+
-- | 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.

 

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

-- Return the result table in any order.

-- The result format is shown in the following example.

 

-- Example 1:

-- 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   |
-- +------------+-----+
-- Explanation: 
-- Teacher 1:
--   - They teach subject 2 in departments 3 and 4.
--   - They teach subject 3 in department 3.
-- Teacher 2:
--   - They teach subject 1 in department 1.
--   - They teach subject 2 in department 1.
--   - They teach subject 3 in department 1.
--   - They teach subject 4 in department 1.


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