Difference between a check constraint and a trigger

Question

Hi, I wanted to know the exact usage of a trigger within a database.
Suppose I want to have a restriction on an insert into a Employees table that provides a check on the time that a particular employee signs in,
I could do that with the help of a before insert trigger that prevents certain values fgrom being inserted.
But cant I do the same thing with the help of a simple check constraint?
Where exactly does the distincton lie between a constraint and a trigger?
Edited by: user8680847 on Sep 24, 2009 11:55 PM

Answer 1

A trigger is fired due to DML operations(Insert,Update,Delete) ot on Database events like startup, shutdown etc. You can write your piece of logic into the trigger to do the necessary operations when the event occurs.
For example, if you want you track the changes made to +(emp)+ table data you can place a before update trigger on the table and place your code inside the triiger to insert the old values into an audit table +(emp_audit)+. Everytime an update is made on the table data the trigger will get fired and will insert the old values of the record into the audit table and the new value in the +(emp)+ table.
A constraint is completely different from a trigger. A constraint is like a validation on the data to be inserted. There are different types of constraints.
I will give you a simple example.
You place the not null constraint on the dept column of the emp table. While you try to insert a new record into the emp table the the dept value cannot be null. Oracle will validate the data you are trying to insert into the table with the constraint and will give you an error if the validation fails.
There are other constraints like:
unique key
foreign key
etc.
Precisely a constraint does not handle a database event as a trigger rather it validates the column data.
Edited by: Priyabrat on Sep 25, 2009 12:27 AM