INSERT Trigger for a rating system: Division by zero error - mysql

Question

I have followed this paper which suggests what is claimed to be a good approach to 5 star rating systems.
The schema is:
CREATE TABLE `ratings` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`product_id` int(11) DEFAULT NULL,
`positive` float NOT NULL DEFAULT 0,
`negative` float NOT NULL DEFAULT 0,
`stars` float DEFAULT 0,
`total` int(11) NOT NULL,
`lower_bound` float DEFAULT NULL,
PRIMARY KEY (`id`)
)
I setup the INSERT trigger which is:
CREATE
TRIGGER `insert_rating` BEFORE INSERT ON `ratings`
FOR EACH ROW
SET new.total = new.positive + new.negative,
new.stars = ROUND( (((new.positive / new.total) * 4) + 1) * 2, 0) / 2,
new.lower_bound = ((new.positive + 1.9208) / (new.positive + new.negative) - 1.96 * SQRT((new.positive * new.negative) / (new.positive + new.negative) + 0.9604) / (new.positive + new.negative)) / (1 + 3.8416 / (new.positive + new.negative))
but upon my first insert of (Also taken from the paper):
INSERT into ratings (product_id, positive, negative) VALUES (1, 0, 0)
I got a Division by zero error. Is there an SQL error in the design of this method or have I done something wrong? Would like to know how to fix.
There's also an update trigger but I don't think I will ever need to update it with zeros:
CREATE
TRIGGER `update_rating` BEFORE UPDATE ON `ratings`
FOR EACH ROW
SET new.total = new.positive + new.negative,
new.stars = ROUND( (((new.positive / new.total) * 4) + 1) * 2, 0) / 2,
new.lower_bound = ((new.positive + 1.9208) / (new.positive + new.negative) - 1.96 * SQRT((new.positive * new.negative) / (new.positive + new.negative) + 0.9604) / (new.positive + new.negative)) / (1 + 3.8416 / (new.positive + new.negative))

Answer 1

Gordon's answer will prevent the error.
The trigger expects either positive or negative to be non-zero, which makes sense. Your insert statement adds NO actual ratings (either positive or negative); so what's the point?
More importantly, why not save yourself some headaches and change the columns to do the work for you?
In your table definition, change the columns to
...
total INT GENERATED ALWAYS AS (positive+negative) STORED,
stars INT GENERATED ALWAYS AS (ROUND((((positive/NULLIF(positive+negative,0))*4)+1) *2, 0)/2) STORED,
lower_bound DOUBLE GENERATED ALWAYS AS ((positive + 1.9208) / NULLIF(positive + negative, 0) - 1.96 * SQRT((positive * negative) / (positive + negative) + 0.9604) / NULLIF(positive + negative, 0))) / (1 + 3.8416 / NULLIF(positive + negative, 0)) STORED,
...
(And remove all the triggers, of course)

Answer 2

The following line sets new.total to 0
SET new.total = new.positive + new.negative
The next line has new.positive / new.total. Since you just set new.total to 0 you get a divide by 0. I would try testing for this condition and setting new.stars = 0 where is happens.

Answer 3

It is like really hard to tell which division is causing the issue. But the solution is NULLIF():
SET new.total = new.positive + new.negative,
new.stars = ROUND( (((new.positive / NULLIF(new.total, 0)) * 4) + 1) * 2, 0) / 2,
n new.lower_bound = ((new.positive + 1.9208) / NULLIF(new.positive + new.negative, 0) - 1.96 * SQRT((new.positive * new.negative) / NULLIF(new.positive + new.negative, 0) + 0.9604) / NULLIF(new.positive + new.negative, 0))) / (1 + 3.8416 / NULLIF(new.positive + new.negative, 0)
This turns 0s into NULLs, so the result is NULL rather than an error.