Question

I’m trying to write a stored function for MySQL to generate a random and unique PIN, which will then be inserted into the ‘PIN’ field in a database table. This has to be done by the DB as various systems may query the database and call this function.

The random number is the easy bit; where I’m struggling is the process for checking that the generated PIN hasn’t already been used and if so, generating new PINs until we get a fresh one. To achieve this, I’m trying to use a WHILE loop but it seems to be getting stuck whirring around because when I run the function MySQL just grinds to a halt and CPU shoots up until I stop/restart the service.

This is my function at the moment:

DELIMITER @@
CREATE FUNCTION generate_unique_pin() RETURNS int(11)
BEGIN

DECLARE pin INT;
DECLARE myloop BOOL;
SET myloop = 1;

WHILE myloop = 1 DO
   SET pin = FLOOR( RAND( ) * 99999999 );
   IF (SELECT COUNT(*) AS used FROM mydb.mytable WHERE PIN = pin) = 0 THEN
      SET myloop = 0;
   END IF;
END WHILE;

RETURN pin;
END@@
DELIMITER ;

I’ve also tried it with something like

SET used = (SELECT COUNT(*) AS used FROM mydb.mytable WHERE PIN = pin)
IF used = 0 THEN (etc...)

But I still get the same issue. The random number code works because if I strip the loop bit out the function works fine (just doesn’t guarantee uniqueness) and the query also works fine if I run it on its own, returning a record set with “used” and 0 or 1.

Any help or advice gratefully received!

Answer 1

You have PIN and pin, a column and a variable with almost identical names, differentiating only by case.

Make them different.