Oracle Trigger How to get Inserted value's parameter - oracle

Question

I have a question while studying Oracle Database's triggers.
First of all, I created two tables like below:
**[Employee]**
(empno NUMBER, VARCHAR2 (20), deptno NUMBER, salary NUMBER)
**[Department]**
(deptno NUMBER, dname VARCHAR2 (20))
Employee deptno refers to Department's deptno.
The trigger I want to create is to calculate the salary average of the data with deptno, such as inserted deptno.
For example, suppose I run the following code.
INSERT INTO employee values (1000, 'PAUL', 10, 4000);
Then Trigger would like to have the following output:
Department [10]'s average of Salary = <avg (salary)>
But I do not know how to pass inserted row's parameter in the Trigger.
Does anyone have a good way for me?

Answer 1

I realized that if I use the AFTER INSERT keyword in the trigger, I get an 'ORA-04091' error because the contents of the table are changing. So I found out that I had to use before insert to get the result.
This is not the correct way, but I was able to express the output I wanted.
It was very simple. Before the table is changed (BEFORE INSERT Keyword), I store the sum of the columns in the variable to get the average.
Then add the new value to the variable using the ': NEW' keyword.
When I used this method, I did not get the 'ORA-04091' error because the table is still before the change.
Thanks to a lot of people, I've found that this is not a good way to write in a trigger.
However, I am thinking about this problem and I have learned a lot of internal actions such as the cost of triggers and when to call them.
Thank you again for all the answerer.

Answer 2

This makes no sense to be implemented as a trigger. This is just a simple SQL query :
select deptno ,avg(salary)
from employee
group by deptno

Answer 3

Trigger is not the best place for such things, but it can be done with compound trigger:
create or replace trigger avg_dept_salary for insert on employee compound trigger
type tb_num is table of number;
v_depts tb_num := tb_num();
v_sum number;
before each row is
begin
if :new.deptno not member of v_depts then
v_depts.extend();
v_depts(v_depts.count()) := :new.deptno;
end if;
end before each row;
after statement is
begin
for i in 1..v_depts.count() loop
select avg(salary) into v_sum from employee where deptno = v_depts(i);
dbms_output.put_line('Department '||v_depts(i)||' average salary: '||v_sum);
end loop;
end after statement;
end;
Test:
insert into employee
select 1001, 'PAUL', 10, 1000 from dual union all
select 1002, 'ANNE', 10, 1700 from dual union all
select 1003, 'MARY', 10, 900 from dual union all
select 1004, 'JOHN', 20, 1200 from dual;
Output for each affected department:
Department 10 average salary: 1900
Department 20 average salary: 1200