How to create ID number without duplication - sql

Question

I have a table called Student with these columns:
First name
Last name
Class ID
Class
I want to create an ID for student, I tried using Row_number Over Partiton but I seem to not be getting what looking for but more of it resetting itself, this is outcome I am trying to get.
First Last Class ClasID ID (outcome looking for)
------------------------------------
John Brown Math M21 ID01
John Brown English E31 ID01
Tom Bank Math M21 ID02
John Brown Gym G41 ID01
Tim Brown English E31 ID03

Answer 1

Use dense_rank():
select t.*, dense_rank() over(order by last, first) as id
from student
This assigns the same id to rows where the first and last names are the same.
If you really want a format like IDnn (which does not makes a lot of sense in my view), then you can format and concatenate:
select t.*,
'ID' || to_char(dense_rank() over(order by last, first), '09') as id
from student
This will fail to generate the proper string if there are more than 99 distinct students.

Answer 2

The dense_rank solution offered by #GMB produces an "id" as part of a given result set, not as an inherent identifier of a given student. The classic method of permanently assigning an ID to an entity (student, employee, class, etc) is to create a SEQUENCE, then an on_insert TRIGGER to populate the ID column with the next_value of the sequence.
SQL> CREATE TABLE STUDENTS
2 (
3 STUDENT_ID NUMBER NOT NULL
4 , FIRST_NAME VARCHAR2(20)
5 , LAST_NAME VARCHAR2(20)
6 , CONSTRAINT STUDENTS_PK PRIMARY KEY
7 (
8 STUDENT_ID
9 )
10 ENABLE
11 );
Table created.
Elapsed: 00:00:00.05
SQL> --
SQL> CREATE SEQUENCE id_seq
2 START WITH 1
3 INCREMENT BY 1
4 ;
Sequence created.
Elapsed: 00:00:00.01
SQL> CREATE OR REPLACE TRIGGER STUDENTS_TRIG
2 BEFORE INSERT ON students
3 FOR EACH ROW
4 BEGIN
5 :new.student_ID := ID_SEQ.NEXTVAL;
6 END;
7 /
Trigger created.
Elapsed: 00:00:00.02
SQL> show errors
No errors.
SQL> --
SQL> insert into students (first_name,
2 last_name)
3 values ('Ed',
4 'Stevens'
5 )
6 ;
1 row created.
Elapsed: 00:00:00.18
SQL> commit;
Commit complete.
Elapsed: 00:00:00.01
SQL> select * from students;
STUDENT_ID FIRST_NAME LAST_NAME
---------- -------------------- --------------------
1 Ed Stevens
1 row selected.
Elapsed: 00:00:00.07
SQL> --
SQL> drop trigger students_trig;
Trigger dropped.
Elapsed: 00:00:00.02
SQL> drop table students purge;
Table dropped.
Elapsed: 00:00:00.05
SQL> drop sequence id_seq;
Sequence dropped.
Elapsed: 00:00:00.01
Starting with version 12, you don't have to create the sequence or trigger, just declare the ID column to be an 'identity' column. I leave it as an exercise for the student to research that concept.