How to update a column of a table depending on the other column automatically in MySQL?

Question

In my table, in one hour, 50 students will submit their marks in some exam everyday.

Following is the table

ID    StudID  Marks  Rank
1     101     56 
2     102     23
3     103     84
4     104     96
5     105     44

I want when all these records are inserted, the rank should get auto calculated and get insterted against each record. For example, Rank column will get filled like this:

ID    StudID  Marks  Rank
1     101     56     3
2     102     23     5
3     103     84     2
4     104     96     1
5     105     44     4

I want to create a single query for this in mysql. Would it be done in a single query or will it require any functions or procedure? How should I write query, function or procedure for above logic in mysql?

Note: I have to implement above logic using PHP.

Answer 1

UPDATE  TableName a
        INNER JOIN
        (
          SELECT  StudID, Marks,
                 (SELECT    COUNT(DISTINCT b.Marks) 
                  FROM  tableName b
                  WHERE a.Marks <= b.Marks
                  ) as rank
          FROM    tableName a
          ORDER   BY Marks DESC
          ) b ON a.StudID = b.StudID
SET a.Rank = b.rank

• SQLFiddle Demo

Answer 2

I assuming the student rerun the insert query, so when the student submit their mark they run query insert into ... Try this:

INSERT INTO <thetable> (ID, StudID, Marks, Rank)
VALUES (<theid>, <thestudentid>, <themark>, NULL)
ON DUPLICATE KEY UPDATE Rank = (SELECT COUNT(*)+1
                                  FROM <thetable>
                                 WHERE Rank IS NOT NULL);

Another assumption: initial value for column "Rank" is NULL , or if empty string change the condition Rank IS NOT NULL to Rank = ''