题目描述：
LeetCode 580. Count Student Number in Departments
A university uses 2 data tables, student and department, to store data about its students and the departments associated with each major.
Write a query to print 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).
Sort your results by descending number of students; if two or more departments have the same number of students, then sort those departments alphabetically by department name.
The student is described as follow:
| Column Name | Type | |--------------|-----------| | student_id | Integer | | student_name | String | | gender | Character | | dept_id | Integer |
where student_id is the student's ID number, student_name is the student's name, gender is their gender, and dept_id is the department ID associated with their declared major.
And the department table is described as below:
| Column Name | Type | |-------------|---------| | dept_id | Integer | | dept_name | String |
where dept_id is the department's ID number and dept_name is the department name.
Here is an example 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 |
The Output should be:
| dept_name | student_number | |-------------|----------------| | Engineering | 2 | | Science | 1 | | Law | 0 |
题目大意：
学生表student，院系表department
求各院系的人数，优先按照人数从大到小排序，其次按照院系名称排序
解题思路：
左连接（LEFT JOIN）
SQL语句：
# Write your MySQL query statement below
SELECT dept_name, IFNULL(student_number, 0) AS student_number
FROM department LEFT JOIN
(SELECT dept_id, count(*) AS student_number
FROM student GROUP BY dept_id) t
USING (dept_id)
ORDER BY student_number DESC, dept_name
本文链接：http://bookshadow.com/weblog/2017/05/09/leetcode-count-student-number-in-departments/
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