Suppose that a database modification results in the execution of a trigger but that trigger generates another modification that happens to violate a primary-key constraint. What happens?

The database can detect only system-defined events.

Explanation:

A trigger is like a stored procedure that Oracle Database invokes automatically whenever a specified event occurs. trigger is like a stored procedure that Oracle Database invokes automatically whenever a specified event occurs.

Both triggers and constraints can constrain data input, but they differ significantly.

A constraint applies to both existing and new data. For example, if a database column has a NOT NULL constraint, then its existing data is NOT NULL and no DML statement can violate the NOT NULL constraint.

A trigger applies only to new data. For example, a trigger can prevent a DML statement from inserting a NULL value into a database column, but the column might contain NULL values that were inserted into the column before the trigger was defined or while the trigger was disabled