Triggers are automatic. They work no matter what caused the data modification—a clerk’s data entry or an application action. A trigger is specific to one or more of the data modification operations (update, insert, and delete), and is executed once for each SQL statement.

For example, to prevent users from removing any publishing companies from the publishers table, you could use this trigger:

create trigger del_pub
on publishers
for delete
as
begin
    rollback transaction
    print "You cannot delete any publishers!"
end

The next time someone tries to remove a row from the publishers table, the del_pub trigger cancels the deletion, rolls back the transaction, and prints a message.

A trigger “fires” only after the data modification statement has completed and Adaptive Server has checked for any datatype, rule, or integrity constraint violation. The trigger and the statement that fires it are treated as a single transaction that can be rolled back from within the trigger. If Adaptive Server detects a severe error, the entire transaction is rolled back.

Triggers are most useful in these situations:

• Triggers can cascade changes through related tables in the database. For example, a delete trigger on the title_id column of the titles table can delete matching rows in other tables, using the title_id column as a unique key to locating rows in titleauthor and roysched.

• Triggers can disallow, or roll back, changes that would violate referential integrity, canceling the attempted data modification transaction. Such a trigger might go into effect when you try to insert a foreign key that does not match its primary key. For example, you could create an insert trigger on titleauthor that rolled back an insert if the new titleauthor.title_id value did not have a matching value in titles.title_id.

• Triggers can enforce restrictions that are much more complex than those that are defined with rules. Unlike rules, triggers can reference columns or database objects. For example, a trigger can roll back updates that attempt to increase a book’s price by more than 1 percent of the advance.

• Triggers can perform simple “what if” analyses. For example, a trigger can compare the state of a table before and after a data modification and take action based on that comparison.