create table emp( empno number primary key 
                  , name varchar2(30) 
                  , sal number )

insert into emp values ( 1000, 'SAM-I-AM', 1500 )

insert into emp values ( 1001, 'MR KNOX', 3000 )

insert into emp values ( 1002, 'FOX IN SOCKS', 3500 )

insert into emp values ( 1003, 'DAISY-HEAD MAYZIE', 4000 )

insert into emp values ( 1004, 'THE LORAX', 5000 )

select * from emp

alter table emp add tax number generated always as (case when sal < 3000 then 0 when sal < 4000 then sal * 0.2 else sal * 0.25 end )

select * from emp

alter table emp drop column tax

alter table emp add tax number

create or replace trigger emp_tax_calc 
     before insert or update on emp 
     for each row 
begin 
    case  
        when :new.sal  < 3000 then :new.tax := 0; 
        when :new.sal  < 4000 then :new.tax := :new.sal * 0.20; 
        else :new.tax := :new.sal * 0.25; 
    end case; 
end; 

update emp set sal = sal

select * from emp