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580. Count Student Number in Departments

中文文档

Description

Table: Student

+--------------+---------+
| Column Name  | Type    |
+--------------+---------+
| student_id   | int     |
| student_name | varchar |
| gender       | varchar |
| dept_id      | int     |
+--------------+---------+
student_id is the primary key (column with unique values) for this table.
dept_id is a foreign key (reference column) to dept_id in the Department tables.
Each row of this table indicates the name of a student, their gender, and the id of their department.

 

Table: Department

+-------------+---------+
| Column Name | Type    |
+-------------+---------+
| dept_id     | int     |
| dept_name   | varchar |
+-------------+---------+
dept_id is the primary key (column with unique values) for this table.
Each row of this table contains the id and the name of a department.

 

Write a solution to report the respective department name and number of students majoring in each department for all departments in the Department table (even ones with no current students).

Return the result table ordered by student_number in descending order. In case of a tie, order them by dept_name alphabetically.

The result format is in the following example.

 

Example 1:

Input: 
Student table:
+------------+--------------+--------+---------+
| student_id | student_name | gender | dept_id |
+------------+--------------+--------+---------+
| 1          | Jack         | M      | 1       |
| 2          | Jane         | F      | 1       |
| 3          | Mark         | M      | 2       |
+------------+--------------+--------+---------+
Department table:
+---------+-------------+
| dept_id | dept_name   |
+---------+-------------+
| 1       | Engineering |
| 2       | Science     |
| 3       | Law         |
+---------+-------------+
Output: 
+-------------+----------------+
| dept_name   | student_number |
+-------------+----------------+
| Engineering | 2              |
| Science     | 1              |
| Law         | 0              |
+-------------+----------------+

Solutions

Solution 1: Left Join + Grouping

We can use a left join to join the Department table and the Student table on dept_id, and then group by dept_id to count the number of students in each department. Finally, we can sort the result by student_number in descending order and dept_name in ascending order.

# Write your MySQL query statement below
SELECT dept_name, COUNT(student_id) AS student_number
FROM
    Department
    LEFT JOIN Student USING (dept_id)
GROUP BY dept_id
ORDER BY 2 DESC, 1;