Can you explain the different types of database triggers in Oracle and their use cases?

Oracle Database provides three types of database triggers:

1. **Row triggers**: These are fired for each row affected by a DML statement (INSERT, UPDATE or DELETE). Row triggers are useful for enforcing complex business rules, data validation and referential integrity constraints.

2. **Statement triggers**: These are fired once for each DML statement. Statement triggers are useful for auditing changes to tables, generating summary reports and logging events.

3. **System Event triggers**: These are fired based on specific system events such as database startup, shutdown or instance startup. System event triggers are useful for managing the environment, automating administrative tasks and handling exceptional conditions.

Here are some examples of each type of trigger:

1. **Row Trigger Example**: Suppose we have a requirement to enforce a business rule that prevents an employee from earning more than their manager. We can implement this using a row trigger on the EMPLOYEES table as follows:

   CREATE OR REPLACE TRIGGER CHECK_EMPLOYEE_SALARY
   BEFORE INSERT OR UPDATE ON EMPLOYEES
   FOR EACH ROW
   DECLARE
     manager_salary NUMBER;
   BEGIN
     SELECT SALARY INTO manager_salary
     FROM EMPLOYEES
     WHERE EMPLOYEE_ID = :NEW.MANAGER_ID;
     
     IF :NEW.SALARY > manager_salary THEN
       RAISE_APPLICATION_ERROR(-20201, 'Employee salary cannot be greater than their manager');
     END IF;
   END;

This trigger fires before a new row is inserted or an existing row is updated in the EMPLOYEES table, and checks if the salary of the employee is greater than the salary of their manager. If it is, an error is raised and the transaction is rolled back.

2. **Statement Trigger Example**: Suppose we have a requirement to log all changes made to the EMPLOYEES table. We can implement this using a statement trigger as follows:

   CREATE OR REPLACE TRIGGER LOG_EMPLOYEE_CHANGES
   AFTER INSERT OR UPDATE OR DELETE ON EMPLOYEES
   DECLARE
     operation VARCHAR2(10);
     timestamp TIMESTAMP;
   BEGIN
     timestamp := SYSTIMESTAMP;
     
     IF INSERTING THEN
       operation := 'INSERT';
     ELSIF UPDATING THEN
       operation := 'UPDATE';
     ELSIF DELETING THEN
       operation := 'DELETE';
     END IF;
     
     INSERT INTO EMPLOYEE_AUDIT (EMPLOYEE_ID, OPERATION, TIME_STAMP)
     VALUES (:OLD.EMPLOYEE_ID, operation, timestamp);
     
   END;

This trigger fires after an insert, update or delete statement is executed on the EMPLOYEES table, and logs the operation, employee ID and timestamp to an AUDIT table.

3. **System Event Trigger Example**: Suppose we have a requirement to startup a background job whenever the database is started. We can implement this using a system event trigger as follows:

   CREATE OR REPLACE TRIGGER START_BACKGROUND_JOB
   AFTER STARTUP ON DATABASE
   BEGIN
     DBMS_SCHEDULER.CREATE_JOB(
       job_name        => 'BG_JOB',
       job_type        => 'PLSQL_BLOCK',
       job_action      => 'BEGIN MY_PACKAGE.RUN_BACKGROUND_PROCESS; END;',
       start_date      => SYSTIMESTAMP,
       repeat_interval => 'FREQ=DAILY; INTERVAL=1',
       enabled         => TRUE
     );
   END;

This trigger fires automatically whenever the database is started, and starts a background job using the DBMS_SCHEDULER package. The background job runs a PL/SQL block that executes a package procedure called MY_PACKAGE.RUN_BACKGROUND_PROCESS, once a day.