MySQL CURSOR loop adds extra pass in stored procedure - mysql

Question

I'm having a problem with a cursor fetch loop in a mysql stored procedure. My stored procedure runs a reordering process which works just fine until the last record of the sort where the order numbering skips a single digit. For example, if I have 10 records and the sort ordering procedure starts at 1, all digits from 1 to 10 should be shown in the resulting records. However, my stored procedure skips the last count, 10 in the case above, and renumbers the final record 11, so the count goes from 9 to 11. This is the case regardless of the number of records involved.
The procedure's logic is fairly simple:
I have a table that holds product type records, with a sort_order column that is used to reorder the records based on usage during a regular batch cycle.
CREATE TABLE `PRODUCT_TYPE` (
`PRODUCT_TYPE_ID` int(11) NOT NULL AUTO_INCREMENT,
`PRODUCT_TYPE_NAME` varchar(45) NOT NULL,
`PRODUCT_CATEGORY_ID` int(11) DEFAULT NULL,
`LIFESPAN_MONTHS` int(11) DEFAULT NULL,
`USER_ID` int(11) DEFAULT NULL,
`UPDATED_BY` int(11) DEFAULT NULL,
`UPDATED_DATE` datetime DEFAULT NULL,
`CREATED_DATE` datetime DEFAULT NULL,
`CREATED_BY` int(11) DEFAULT NULL,
`REVIEWED` bit(1) NOT NULL DEFAULT b'0',
`SORT_ORDER` int(11) NOT NULL DEFAULT '0',
PRIMARY KEY (`PRODUCT_TYPE_ID`),
KEY `fk_PRODUCT_TYPE_PRODUCT_CATEGORY1_idx` (`PRODUCT_CATEGORY_ID`),
KEY `fk_PRODUCT_TYPE_USERS1_idx` (`USER_ID`),
KEY `fk_PRODUCT_TYPE_USERS2_idx` (`UPDATED_BY`),
KEY `fk_PRODUCT_TYPE_USERS3_idx` (`CREATED_BY`),
CONSTRAINT `fk_PRODUCT_TYPE_PRODUCT_CATEGORY1` FOREIGN KEY (`PRODUCT_CATEGORY_ID`) REFERENCES `PRODUCT_CATEGORY` (`PRODUCT_CATEGORY_ID`) ON DELETE NO ACTION ON UPDATE NO ACTION,
CONSTRAINT `fk_PRODUCT_TYPE_USERS1` FOREIGN KEY (`USER_ID`) REFERENCES `USERS` (`USER_ID`) ON DELETE NO ACTION ON UPDATE NO ACTION,
CONSTRAINT `fk_PRODUCT_TYPE_USERS2` FOREIGN KEY (`UPDATED_BY`) REFERENCES `USERS` (`USER_ID`) ON DELETE NO ACTION ON UPDATE NO ACTION,
CONSTRAINT `fk_PRODUCT_TYPE_USERS3` FOREIGN KEY (`CREATED_BY`) REFERENCES `USERS` (`USER_ID`) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=InnoDB AUTO_INCREMENT=61 DEFAULT CHARSET=latin1;
I run the following stored procedure on a nightly basis to reorder the product type records based on the number of references to each type using the sort_order column to record the order.
DELIMITER $$
CREATE DEFINER=`root`#`localhost` PROCEDURE `REORDER_MANUFACTURERS`()
BEGIN
DECLARE DONE BOOL;
DECLARE MID INT;
DECLARE MNAME VARCHAR(255);
DECLARE USES INT;
DECLARE SORT_ORDER_COUNTER INT;
DECLARE CUR CURSOR FOR SELECT M.MANUFACTURER_ID, M.MANUFACTURER_NAME, COUNT(U.UNIT_ID) AS USES
FROM MANUFACTURERS M LEFT JOIN mydb.UNITS U ON M.MANUFACTURER_ID = U.MANUFACTURER_ID
GROUP BY M.MANUFACTURER_ID, M.MANUFACTURER_NAME
ORDER BY USES DESC, MANUFACTURER_NAME;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET DONE = TRUE;
SET SORT_ORDER_COUNTER = 0;
OPEN CUR;
READ_LOOP: LOOP
FETCH CUR INTO MID, MNAME, USES;
UPDATE MANUFACTURERS SET SORT_ORDER = SORT_ORDER_COUNTER WHERE MANUFACTURER_ID = MID;
IF DONE THEN
LEAVE READ_LOOP;
END IF;
SET SORT_ORDER_COUNTER = SORT_ORDER_COUNTER + 1;
END LOOP;
CLOSE CUR;
END
For the life of me, I can't find a problem with this logic that would cause the count to skip a beat. Any help would be appreciated.

Answer 1

Change:
FETCH CUR INTO MID, MNAME, USES;
UPDATE MANUFACTURERS SET SORT_ORDER = SORT_ORDER_COUNTER
WHERE MANUFACTURER_ID = MID;
IF DONE THEN
LEAVE READ_LOOP;
END IF;
To:
FETCH CUR INTO MID, MNAME, USES;
IF DONE THEN
LEAVE READ_LOOP;
END IF;
UPDATE MANUFACTURERS SET SORT_ORDER = SORT_ORDER_COUNTER
WHERE MANUFACTURER_ID = MID;
It is because:
If you FETCH past the last row in the result set, the values of the target fields or variables are indeterminate and the NOTFOUND attribute returns TRUE.
Refer to: (This is on Oracle cursors, but applicable to others as well):
Oracle: Fetch Statement