Check restriction for mysql

Mysql does not suppot for check restriction but oracle does. Here using trigger we can achive the same restrction as check does.

Scenario:

Here is a IDcard field in table student with a restriction that the length of IDCrad must less than 18.

4 parameters we can use:

1. trigger-name: credit_tri
2. target-table:student
3. target-field:IDCard
4. error-msg:"length is above 18,cannot insert"

mysql> delimiter $
mysql> create trigger credit_tri before insert
    -> on student for each row
    -> begin
    ->     declare msg varchar(200);
    ->     if (new.IDCard > 18) then
    ->         set msg = "length is above 18. Cannot insert.";
    ->             signal sqlstate ‘HY000‘ SET message_text = msg;
    ->     end if;
    -> end $