查询50号部门员工的平均工资，如果平均工资低于5000，则每个员工工资增加1000

declare

  v_avgsal employees.salary%TYPE;
begin
  select avg(salary) into v_avgsal from empolyees where department_id = 50;
  if v_avgsal < 5000 then
    update employees set salary = salary + 1000 where department_id = 50;
  end if;
end;