mysql trigger does not insert selected columns - mysql

Question

CREATE DEFINER=`ir55`#`%` TRIGGER UpdateAverageBookRating
AFTER INSERT
ON Reviews FOR EACH ROW
BEGIN
DECLARE AverageRating double;
DECLARE lastbookid int;
SELECT BookID FROM Reviews WHERE ReviewID = LAST_INSERT_ID() INTO lastbookid;
SELECT AVG(Rating) FROM Reviews WHERE BookID = lastbookid INTO AverageRating;
UPDATE CentralCatelogue SET Rating = AverageRating WHERE BookID = lastbookid;
END
The above trigger does not update the Rating column in CentralCatelogue because the value of Rating and BookID are empty.
Not sure why, I tried to update the Rating with static values and it works, I believe there is something wrong with selecting the columns in 'Reviews' and assigning them into variables.
Appreciate your help.
Thanks

Answer 1

If your schema looks a bit like this
DROP TABLE REVIEWS,CENTRALCATALOGUE;
CREATE TABLE `reviews` (
`bookid` int(11) DEFAULT NULL,
`rating` int(11) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
CREATE TABLE `centralcatalogue` (
`bookid` int(11) DEFAULT NULL,
`rating` double(10,2) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
DROP TRIGGER IF EXISTS T;
DELIMITER $$
CREATE TRIGGER `t` AFTER INSERT ON `reviews` FOR EACH ROW BEGIN
DECLARE AverageRating double(10,2);
if not exists (select 1 from centralcatalogue where bookid = new.bookid) then
insert into debug_table(msg) values (concat('inserting:',new.bookid,':',new.rating));
insert into centralcatalogue values(new.bookid,new.rating);
else
SELECT AVG(Rating) FROM Reviews WHERE BookID = new.bookid INTO AverageRating;
insert into debug_table(msg) values (concat('updating',new.bookid,':',averagerating));
UPDATE CentralCatalogue SET Rating = AverageRating WHERE BookID = new.bookid;
end if;
END $$
delimiter ;
Note 1) the test and insert to see if bookid exists in centralcatalogue 2) The debug_table to assist debugging 3) changed averagerating to double(10,2) to avoid rounding up
the insert
insert into reviews values(1,1),(1,2),(2,3);
produces
+--------+--------+
| bookid | rating |
+--------+--------+
| 1 | 1.50 |
| 2 | 3.00 |
+--------+--------+
2 rows in set (0.02 sec)
BTW I don't really like this trigger since it tests and calculates for each row. It might be better if you encapsulated the insert and update in a transaction.