리트코드 : 2356. Number of Unique Subjects Taught by Each Teacher

리트코드 : 2356. Number of Unique Subjects Taught by Each Teacher

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문제

Table: Teacher

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

• 각 선생 별 과목 수 구하기

문제 풀이

MySQL

SELECT teacher_id, COUNT(DISTINCT subject_id) as cnt
FROM teacher
GROUP BY teacher_id

• GRUOP BY + COUNT DISTINCT

Pandas

import pandas as pd

def count_unique_subjects(teacher: pd.DataFrame) -> pd.DataFrame:
    grouped = teacher.groupby('teacher_id').agg(
        cnt = ('subject_id','nunique')
    ).reset_index()
    return grouped

• groupby + unique

코멘트

• 기본문제