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High School Attendance

Problem

Table: Students

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| student_id    | int     |
| student_name  | varchar |
| grade         | int     |
+---------------+---------+
student_id is the primary key for this table.
This table contains the information of students in a high school.

Table: Attendance

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| student_id    | int     |
| attendance_date | date  |
| status        | enum    |
+---------------+---------+
(student_id, attendance_date) is the primary key for this table.
status is an ENUM of type ('Present', 'Absent').

Problem Definition

Write a solution to find the students who were absent for three or more consecutive days.

Return the result table ordered by student_id and start_date, where start_date is the first date of the consecutive absences.

Example

Input: 
Students table:
+------------+--------------+-------+
| student_id | student_name | grade |
+------------+--------------+-------+
| 1          | Alice        | 10    |
| 2          | Bob          | 11    |
| 3          | Charlie      | 10    |
+------------+--------------+-------+

Attendance table:
+------------+-----------------+--------+
| student_id | attendance_date | status |
+------------+-----------------+--------+
| 1          | 2020-11-01      | Absent |
| 1          | 2020-11-02      | Absent |
| 1          | 2020-11-03      | Absent |
| 2          | 2020-11-01      | Present|
| 2          | 2020-11-02      | Absent |
| 2          | 2020-11-03      | Absent |
| 2          | 2020-11-04      | Absent |
| 3          | 2020-11-01      | Present|
| 3          | 2020-11-02      | Present|
+------------+-----------------+--------+

Output: 
+------------+-------------+
| student_id | start_date  |
+------------+-------------+
| 1          | 2020-11-01  |
| 2          | 2020-11-02  |
+------------+-------------+

Try It Yourself

java
-- TODO: Write your user queries here

Solution

To identify students who were absent for three or more consecutive days, we can utilize SQL's window functions and Common Table Expressions (CTEs). This approach allows us to efficiently analyze attendance records, detect consecutive absences, and extract the required information for each qualifying student.

SQL Query

WITH RankedAttendance AS (
    SELECT 
        student_id, 
        attendance_date, 
        status,
        ROW_NUMBER() OVER (PARTITION BY student_id ORDER BY attendance_date) AS s_rank
    FROM Attendance
    WHERE status = 'Absent'
),
ConsecutiveAttendance AS (
    SELECT 
        student_id, 
        MIN(attendance_date) AS start_date,
        COUNT(*) AS consecutive_days
    FROM (
        SELECT 
            student_id, 
            attendance_date,
            s_rank - ROW_NUMBER() OVER (PARTITION BY student_id ORDER BY attendance_date) AS grouping_col
        FROM RankedAttendance
    ) AS grouped_absences
    GROUP BY student_id, grouping_col
    HAVING COUNT(*) >= 3
)
SELECT student_id, start_date
FROM ConsecutiveAttendance
ORDER BY student_id, start_date;

Step-by-Step Approach

Step 1: Filter Absent Records and Assign Sequential Ranks

Extract all absence records and assign a sequential rank to each absence per student based on the attendance_date. This ranking facilitates the detection of consecutive absences.

SQL Query:

WITH RankedAttendance AS (
    SELECT 
        student_id, 
        attendance_date, 
        status,
        ROW_NUMBER() OVER (PARTITION BY student_id ORDER BY attendance_date) AS s_rank
    FROM Attendance
    WHERE status = 'Absent'
)

Explanation:

Output After Step 1:

Assuming the example input provided, the RankedAttendance CTE would produce:

+------------+-----------------+--------+--------+
| student_id | attendance_date | status | s_rank |
+------------+-----------------+--------+--------+
| 1          | 2020-11-01      | Absent | 1      |
| 1          | 2020-11-02      | Absent | 2      |
| 1          | 2020-11-03      | Absent | 3      |
| 2          | 2020-11-02      | Absent | 1      |
| 2          | 2020-11-03      | Absent | 2      |
| 2          | 2020-11-04      | Absent | 3      |
+------------+-----------------+--------+--------+

Step 2: Calculate Grouping Identifier to Detect Consecutive Absences

Compute a grouping identifier (grouping_col) to help group consecutive absences. By subtracting the row number from the sequential rank, consecutive dates will have the same grouping_col.

SQL Query:

ConsecutiveAttendance AS (
    SELECT 
        student_id, 
        MIN(attendance_date) AS start_date,
        COUNT(*) AS consecutive_days
    FROM (
        SELECT 
            student_id, 
            attendance_date,
            s_rank - ROW_NUMBER() OVER (PARTITION BY student_id ORDER BY attendance_date) AS grouping_col
        FROM RankedAttendance
    ) AS grouped_absences
    GROUP BY student_id, grouping_col
    HAVING COUNT(*) >= 3
)

Explanation:

Output After Step 2:

Based on the example input, the ConsecutiveAttendance CTE would produce:

+------------+-------------+-----------------+
| student_id | start_date  | consecutive_days|
+------------+-------------+-----------------+
| 1          | 2020-11-01  | 3               |
| 2          | 2020-11-02  | 3               |
+------------+-------------+-----------------+

Step 3: Select and Order the Final Results

Retrieve the student_id and start_date of each qualifying student who was absent for three or more consecutive days. Order the results by student_id and start_date.

SQL Query:

SELECT student_id, start_date
FROM ConsecutiveAttendance
ORDER BY student_id, start_date;

Explanation:

Final Output:

+------------+-------------+
| student_id | start_date  |
+------------+-------------+
| 1          | 2020-11-01  |
| 2          | 2020-11-02  |
+------------+-------------+
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