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The events that execute triggers are database transactions, triggers can be executed immediately BEFORE or AFTER the statements INSERTs, UPDATEs, DELETEs.• AFTER row-level triggers are frequently used in auditing applications, since they do not fire until the row has been modified.An Example of Trigger -- for the table major_stats(major, total_credits, total_students); CREATE OR REPLACE TRIGGER update Major Stats AFTER INSERT OR DELETE OR UPDATE ON students -- Oracle will check the status of this table DECLARE -- unlike a procedure, use DECLARE keyword CURSOR c_statistics IS SELECT major, COUNT(*) total_students, SUM(current_credits) total_credits FROM students GROUP BY major; BEGIN FOR v_stats Record IN c_statistics LOOP UPDATE major_stats SET total_credits = v_stats Record.total_credits, total_students = v_stats Record.total_students WHERE major = v_stats Record.major; IF SQL%NOTFOUND THEN INSERT INTO major_stats(major, total_credits, total_students) VALUES(v_stats Record.major, v_stats Record.total_credits, v_stats Record.total_students); END IF; END LOOP; END update Major Stats; -- :old and :new, e.g., :new.field or :old.field. • :old refers to the data as it existed prior to the transaction. The :new values are the data values that the transaction creates (such as the columns in an inserted record).• :old is undefined for INSERT statements, :new is undefined for DELETE statements.For example, they cannot be assigned as entire records, e.g., v_tmprec := :old; is not legal.Only the individual fields within them may be assigned, e.g., v_:= :old.col;.Clearly, there is a great deal of flexibility in the design of a trigger.
• Maintaining complex integrity constraints (referential integrity) or business rules • Auditing information in a table by recording the changes.• Likewise, :old and :new cannot be passed to procedures or functions that take arguments of triggering_table%ROWTYPE, since the actual parameter is assigned to the formal parameter in the same way as a standard assignment.For example, the Generate Student ID trigger shown next uses :new.CREATE [OR REPLACE] TRIGGER trigger_name ON table_name [FOR EACH ROW] [WHEN (logical_expression)] [DECLARE] [PRAGMA AUTONOMOUS_TRANSACTION;] declaration_statements; BEGIN execution_statements; END [trigger_name]; / CREATE or REPLACE TRIGGER audit_emp_hours BEFORE INSERT OR UPDATE OR DELETE ON employees BEGIN IF TO_CHAR (SYSDATE, 'HH24: MI') NOT BETWEEN '' AND '' OR TO_CHAR (SYSDATE, 'DY') IN ('SAT', 'SUN') THEN RAISE_APPLICATION_ERROR (-20205, 'You may only make changes during normal office hours'); END IF; END; / Trigger Created.SQL update employees set salary=43000 where employee_id=210; update employees set salary=43000 where employee_id=210 * ERROR at line 1: ORA-20205: You may only make changes during normal office hours ORA-06512: at "HR.