trigger in PL/SQL - sql

Question

There are two tables given:
1)
employee(eno,ename,basic,da,gross)
da=basic*(5.0/100)
gross = basic+da
2)
sal_hist(eno, sys_dt, old_basic)
How to write a trigger to update the 'da' and 'gross' whenever I am updating basic salary of the employee?

Answer 1

PL/SQL tag suggests that you use Oracle database.
You said that there's yet another table, sal_hist, but - you didn't say what to do with it. I presume you'd want to save the old basic salary.
In that case, trigger would look like this:
SQL> create or replace trigger trg_biu_emp
2 before insert or update on employee
3 for each row
4 begin
5 insert into sal_hist(eno, sys_dt, old_basic) values
6 (:new.eno, sysdate, :old.basic);
7
8 :new.da := :new.basic * 5 / 100;
9 :new.gross := :new.basic + :new.da;
10 end;
11 /
Trigger created.
Let's see how it works:
SQL> select * From employee;
ENO ENAME BASIC DA GROSS
---------- ----- ---------- ---------- ----------
1 Scott 100 0 0
2 Tiger 500 0 0
SQL> select * From sal_hist;
no rows selected
SQL> update employee set basic = 200 where eno = 1;
1 row updated.
SQL> insert into employee (eno, ename, basic) values (3, 'King', 1000);
1 row created.
SQL> select * From employee;
ENO ENAME BASIC DA GROSS
---------- ----- ---------- ---------- ----------
1 Scott 200 10 210
2 Tiger 500 0 0
3 King 1000 50 1050
SQL> select * From sal_hist;
ENO SYS_DT OLD_BASIC
---------- ------------------- ----------
1 06.06.2020 11:10:49 100
3 06.06.2020 11:12:07
SQL>

Answer 2

CREATE DEFINER=`root`#`localhost` TRIGGER `TRIGGERNAME` AFTER UPDATE ON `employee` FOR EACH ROW
BEGIN
SET employee.da = employee.basic*(5.0/100)
SET employee.gross = employee.basic + (employee.basic*(5.0/100))
END

Answer 3

The way you have them defined both da and gross are derivable directly from base. However as a standard column I can update either of them directly. If this is not desirable, then you can declare them as virtual columns.
alter table employee drop (da, gross);
alter table employee add (
da generated always as base * 0.05 virtual
, gross generated always as base * 1.05 virtual
);
Now neither column can be updated independently and both are automatically updated when base is updated. You use them as normal columns of select, but DO NOT reference then in Insert or Update. And there is NO trigger needed.
See fiddle. Note: You did not specify what Oracle version you are using. This requires 11gR1 or higher.