Question

I am writing a trigger that will execute everytime a student completes an exam assignment and will update a table with what their current rank is amongst their fellow students (highest marks gives rank of 1, etc)

I started with the following query to see if I would get usable data from my student exam submissions

SET @rownum := 0;
SET @school := 'hillview';
SELECT userID, rank, firstname, institution
FROM (
SELECT @rownum := @rownum + 1 AS rank, userID, firstname, institution FROM(
    SELECT userID, sum(marks), firstname, institution
    FROM competition_history
    WHERE institution = @school
    GROUP BY userID
    ORDER BY marks DESC) as inner_rank
) as r
WHERE userID = 2;

Which returned a correct result of

++++++++++++++++++++++++
uid|rank|fname |school
++++++++++++++++++++++++
2 | 2 |stefan|hillview

So my student with the userID = 2 is ranked second, great, now I want to convert this into a trigger and write it to a table that stores just userID and rank

I tried to convert this to a trigger (and in so removing the hardcoded institution variable)

DELIMITER $$
DROP TRIGGER IF EXISTS CallRankProc;
CREATE TRIGGER callRankProc AFTER INSERT
ON competition_history
FOR EACH ROW
BEGIN
DECLARE rownum INTEGER DEFAULT 0;
DECLARE userRank INTEGER;
SELECT _rank into userRank
FROM (
SELECT rownum = rownum + 1 AS _rank, userID, firstname, institution FROM(
    SELECT userID, sum(marks), firstname, institution
    FROM competition_history
    WHERE institution = NEW.institution
    GROUP BY userID
    ORDER BY marks DESC) as inner_rank
) as r
WHERE userID = NEW.userID;

INSERT INTO `student_ranks`(`userID`,`rank`) VALUES (NEW.userID,userRank)
ON DUPLICATE KEY UPDATE rank = userRank;

END$$

However, this inserts into my table
+++++++++++
uid| rank |
+++++++++++
2 | 0 |

Now since it inserted uid = 2 I believe it is pulling the correct values, and I can’t seem to figure out why then the userRank is 0, meaning that rownum is not incrementing.

I am trying to pinpoint why the trigger code returns the default variable value and if it is based on the nested select. Can anyone pinpoint the problem? Can you have nested selects in MySQL triggers? When I added the trigger no errors were thrown.

Answer 1

Try to set rownum to a default value

DECLARE rownum INTEGER DEFAULT 0

in your current trigger, the following expression

rownum = rownum + 1

will evaluate to NULL, due to the default value of rownum being null.

Additionally the = operator checks for equality (as in “x is equal to y”) which means the expression will evaluate to true or false. And since rownum will never be equal to rownum+1 , your result will be false (this is why you are seeing a 0 in your resultset).

EDIT
Try SELECT rownum+1 INTO rownum