Compound Triggers , Avoid Mutating-Table Error

Using Compound Triggers to Avoid Mutating-Table Error

You can use compound triggers to avoid the mutating-table error (ORA-04091) described in Trigger Restrictions on Mutating Tables.

Scenario: A business rule states that an employee's salary increase must not exceed 10% of the average salary for the employee's department. This rule must be enforced by a trigger.

Solution: Define a compound trigger on updates of the table hr.employees, as in Example 9-4. The state variables are initialized each time the trigger fires (even when the triggering statement is interrupted and restarted).

Example 9-4 Compound Trigger that Avoids Mutating-Table Error

CREATE OR REPLACE TRIGGER Check_Employee_Salary_Raise
  FOR UPDATE OF Salary ON Employees
COMPOUND TRIGGER
  Ten_Percent                 CONSTANT NUMBER := 0.1;
  TYPE Salaries_t             IS TABLE OF Employees.Salary%TYPE;
  Avg_Salaries                Salaries_t;
  TYPE Department_IDs_t       IS TABLE OF Employees.Department_ID%TYPE;
  Department_IDs              Department_IDs_t;

  TYPE Department_Salaries_t  IS TABLE OF Employees.Salary%TYPE
                                INDEX BY VARCHAR2(80);
  Department_Avg_Salaries     Department_Salaries_t;

  BEFORE STATEMENT IS
  BEGIN
    SELECT               AVG(e.Salary), NVL(e.Department_ID, -1)
      BULK COLLECT INTO  Avg_Salaries, Department_IDs
      FROM               Employees e
      GROUP BY           e.Department_ID;
    FOR j IN 1..Department_IDs.COUNT() LOOP
      Department_Avg_Salaries(Department_IDs(j)) := Avg_Salaries(j);
    END LOOP;
  END BEFORE STATEMENT;

  AFTER EACH ROW IS
  BEGIN
    IF :NEW.Salary - :Old.Salary >
      Ten_Percent*Department_Avg_Salaries(:NEW.Department_ID)
    THEN
      Raise_Application_Error(-20000, 'Raise too big');
    END IF;
  END AFTER EACH ROW;
END Check_Employee_Salary_Raise;