mysql triggers simulating assertions - mysql

Question

Let’s consider table
Video(
IDvideo(PK),
Date,
Description,
User
)
with mysql I have no way of writing assertions.
Is it possible to simulate the following assertion using one or more triggers ?
create assertion asser1
check (0 =
( select count(*)
from Video
where Date >= DATE_SUB(current_date(),INTERVAL 1 YEAR )
&& Date<=current_date()
group by User
having count(*) > 200
)
)
how should I write that trigger?

Answer 1

Well, the problem is that MySQL doesn't have an equivalent of a STOP ACTION command. So basically, the work arounds are quite dirty:
One way is that you can violate a constraint inside the trigger to bubble an error and cancel the insert:
CREATE TABLE stop_action (
id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
name VARCHAR(35),
UNIQUE KEY (id, name)
);
INSERT INTO stop_action (1, 'Assert Failure');
Then, in the trigger, just try to:
INSERT INTO stop_action (1, 'Assert Failure');
The benefit of that, is that the error that's returned will be a duplicate key error, and the text will include "Assert Failure".
So then your trigger would become:
delimiter |
CREATE TRIGGER asser1_before BEFORE INSERT ON test1
FOR EACH ROW BEGIN
SELECT count(*) INTO test FROM (select count(*)
from Video
where Date >= DATE_SUB(current_date(),INTERVAL 1 YEAR )
&& Date<=current_date()
group by User
having count(*) > 200);
IF test != 0 THEN
INSERT INTO stop_action (1, 'Assert Failure');
END IF;
END;
|
delimiter ;
Now, you'd need to do this before UPDATE as well, otherwise you could update the date into an invalid state. But otherwise, that should at least get you started...