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2346. Compute the Rank as a Percentage

Description

Table: Students

+---------------+------+
| Column Name   | Type |
+---------------+------+
| student_id    | int  |
| department_id | int  |
| mark          | int  |
+---------------+------+
student_id is the primary key of this table.
Each row of this table indicates a student's ID, the ID of the department in which the student enrolled, and their mark in the exam.

 

Write an SQL query that reports the rank of each student in their department as a percentage, where the rank as a percentage is computed using the following formula: (student_rank_in_the_department - 1) * 100 / (the_number_of_students_in_the_department - 1). The percentage should be rounded to 2 decimal places. student_rank_in_the_department is determined by descending mark, such that the student with the highest mark is rank 1. If two students get the same mark, they also get the same rank.

Return the result table in any order.

The query result format is in the following example.

 

Example 1:

Input: 
Students table:
+------------+---------------+------+
| student_id | department_id | mark |
+------------+---------------+------+
| 2          | 2             | 650  |
| 8          | 2             | 650  |
| 7          | 1             | 920  |
| 1          | 1             | 610  |
| 3          | 1             | 530  |
+------------+---------------+------+
Output: 
+------------+---------------+------------+
| student_id | department_id | percentage |
+------------+---------------+------------+
| 7          | 1             | 0.0        |
| 1          | 1             | 50.0       |
| 3          | 1             | 100.0      |
| 2          | 2             | 0.0        |
| 8          | 2             | 0.0        |
+------------+---------------+------------+
Explanation: 
For Department 1:
 - Student 7: percentage = (1 - 1) * 100 / (3 - 1) = 0.0
 - Student 1: percentage = (2 - 1) * 100 / (3 - 1) = 50.0
 - Student 3: percentage = (3 - 1) * 100 / (3 - 1) = 100.0
For Department 2:
 - Student 2: percentage = (1 - 1) * 100 / (2 - 1) = 0.0
 - Student 8: percentage = (1 - 1) * 100 / (2 - 1) = 0.0

Solutions

  • # Write your MySQL query statement below
SELECT
    student_id,
    department_id,
    ifnull(
        round(
            (
                rank() OVER (
                    PARTITION BY department_id
                    ORDER BY mark DESC
                ) - 1
            ) * 100 / (count(1) OVER (PARTITION BY department_id) - 1),
            2
        ),
        0
    ) AS percentage
FROM Students;

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