Question

I have a table that looks like this:

studentID | subjectID | attendanceStatus | classDate  | classTime | lecturerID |
12345678    1234        1                  2012-06-05   15:30:00
87654321
12345678    1234        0                  2012-06-08   02:30:00

I want a query that reports if a student has been absent for 3 or more consecutive classes. based on studentID and a specific subject between 2 specific dates as well. Each class can have a different time. The schema for that table is:

PK(`studentID`, `classDate`, `classTime`, `subjectID, `lecturerID`)

Attendance Status: 1 = Present, 0 = Absent

Edit: Worded question so that it is more accurate and really describes what was my intention.

Answer 1

I wasn’t able to create an SQL query for this. So instead, I tried a PHP solution:

1. Select all rows from table, ordered by student, subject and date
2. Create a running counter for absents, initialized to 0
3. Iterate over each record:
   • If student and/or subject is different from previous row
     • Reset the counter to 0 (present) or 1 (absent)
   • Else, that is when student and subject are same
     • Set the counter to 0 (present) or plus 1 (absent)

I then realized that this logic can easily be implemented using MySQL variables, so:

SET @studentID = 0;
SET @subjectID = 0;
SET @absentRun = 0;

SELECT *,
CASE
    WHEN (@studentID  = studentID) AND (@subjectID  = subjectID) THEN @absentRun := IF(attendanceStatus = 1, 0, @absentRun + 1)
    WHEN (@studentID := studentID) AND (@subjectID := subjectID) THEN @absentRun := IF(attendanceStatus = 1, 0, 1)
END AS absentRun
FROM table4
ORDER BY studentID, subjectID, classDate

You can probably nest this query inside another query that selects records where absentRun >= 3.

SQL Fiddle