LeetCode--580. 统计各专业学生人数
文章目录
- 1 题目描述
- 2 测试用例
- 3 解题思路
1 题目描述
表: Student
+--------------+---------+
| Column Name | Type |
+--------------+---------+
| student_id | int |
| student_name | varchar |
| gender | varchar |
| dept_id | int |
+--------------+---------+
student_id 是该表的主键 (具有唯一值的列)
dept_id 是 Department 表中 dept_id 的外键
该表的每一行都表示学生的姓名, 性别和所属系的 id
表: Department
+-------------+---------+
| Column Name | Type |
+-------------+---------+
| dept_id | int |
| dept_name | varchar |
+-------------+---------+
dept_id 是该表的主键 (具有唯一值的列)
该表的每一行包含一个部门的 id 和名称
编写解决方案, 为 Department 表中的所有部门 (甚至是没有当前学生的部门) 报告各自的部门名称和每个部门的学生人数
按 student_number 降序 返回结果表. 如果是平局, 则按 dept_name 的 字母顺序 排序
2 测试用例
输入:
Student 表:
+------------+--------------+--------+---------+
| student_id | student_name | gender | dept_id |
+------------+--------------+--------+---------+
| 1 | Jack | M | 1 |
| 2 | Jane | F | 1 |
| 3 | Mark | M | 2 |
+------------+--------------+--------+---------+
Department 表:
+---------+-------------+
| dept_id | dept_name |
+---------+-------------+
| 1 | Engineering |
| 2 | Science |
| 3 | Law |
+---------+-------------+
输出:
+-------------+----------------+
| dept_name | student_number |
+-------------+----------------+
| Engineering | 2 |
| Science | 1 |
| Law | 0 |
+-------------+----------------+
3 解题思路
- 用
Department左外连接Student, 按照dept_name分组统计学生数量
select d.dept_name, count(s.student_id) as student_number
from Department as d
left join Student as s on d.dept_id = s.dept_id
group by d.dept_name;
- 对
student_number倒序排序, 如果是平局, 则按dept_name的字母顺序排序
select d.dept_name, count(s.student_id) as student_number
from Department as d
left join Student as s on d.dept_id = s.dept_id
group by d.dept_name
order by student_number desc, d.dept_name;
查询结果
+-----------+--------------+
|dept_name |student_number|
+-----------+--------------+
|Engineering|2 |
|Science |1 |
|Law |0 |
+-----------+--------------+