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?

解决方案

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...