Should I use a sequence?

Question

Hi,
I would like to achieve the following
h5. table A
record #1
--column 1 (number): 1 (pk)
--column 2 (char): AAR
record #2
--column 1 (number): 2 (pk)
--column 2 (char): GGT
h5. table B
column 1 (number): 1 (pk)
column 2 (number): 1 (fk on table A)
column 3 (char): AAR-1
The idea is that column 3 on table B is automatically generated by the application., and gets the value according to the FK to table A
Therefore, the next record created in table B, if having column 2 value as "1" (from table A) then column 3 should get value "AAR-2"
Instead if in table B a record is inserted pointing to column 2 values "2", then column 3 would get GGT-01.
What do you recommend? To have an additional column (number) in table A starting with 0, and every time a new record is inserted in table B I should get the max(id) and increment by 1 after creating the record. Or would it be reasonable to create a sequence for each record table A (assuming that a table will never grow to more than 500 records)?
Thanks for your help

Answer 1

In the below example, I am not using sequence; I used trigger which works for PK/FK and sequence simultaneously : Windows XP and Enterprise Edition 11.2.0.1
create table tablea(car_code number,car_name varchar2(50));
insert into tablea values (1,'Audi');
insert into tablea values (2,'Mercedes');
insert into tablea values (3,'Honda');
insert into tablea values (4,'Toyota');
insert into tablea values (5,'Maruti');
create table tableb(person_name varchar2(40),his_car_id number,his_car_name varchar2(60));
create or replace trigger trigb
before insert on tableb
for each row
declare
his_car_name varchar(60);
his_car_id number;
begin
select car_code into his_car_id from tablea where car_code=:NEW.his_car_id;
if his_car_id is null then
raise_application_error(-20002,'Invalid Car');
end if;
select max(his_car_name) into his_car_name from tableb where his_car_id=:NEW.his_car_id;
if his_car_name is null then
select car_name into his_car_name from tablea where car_code=:NEW.his_car_id;
his_car_name := his_car_name || '-0000';
end if;
:NEW.his_car_name := substr(his_car_name,1,instr(his_car_name,'-')-1) || '-' || to_char(nvl(substr(his_car_name,instr(his_car_name,'-')+1,60),0)+1,'FM0000');
end trigb;
SQL> select * from tablea;
  CAR_CODE CAR_NAME
         1 Audi
         2 Mercedes
         3 Honda
         4 Toyota
         5 Maruti
SQL> select * from tableb;
no rows selected
SQL> insert into tableb (person_name,his_car_id) values ('Scott',3);
1 row created.
SQL> insert into tableb (person_name,his_car_id) values ('John',3);
1 row created.
SQL> insert into tableb (person_name,his_car_id) values ('Mike',1);
1 row created.
SQL> insert into tableb (person_name,his_car_id) values ('Smith',4);
1 row created.
SQL> insert into tableb (person_name,his_car_id) values ('Flower',2);
1 row created.
SQL> insert into tableb (person_name,his_car_id) values ('Lewis',1);
1 row created.
SQL> insert into tableb (person_name,his_car_id) values ('Ponting',4);
1 row created.
SQL> insert into tableb (person_name,his_car_id) values ('Bill',1);
1 row created.
SQL> insert into tableb (person_name,his_car_id) values ('Tony',11);
insert into tableb (person_name,his_car_id) values ('Tony',11)
ERROR at line 1:
ORA-01403: no data found
ORA-06512: at "SCOTT.TRIGB", line 5
ORA-04088: error during execution of trigger 'SCOTT.TRIGB'
SQL> select * from tableb;
PERSON_NAME                              HIS_CAR_ID HIS_CAR_NAME
Scott                                             3 Honda-0001
John                                              3 Honda-0002
Mike                                              1 Audi-0001
Smith                                             4 Toyota-0001
Flower                                            2 Mercedes-0001
Lewis                                             1 Audi-0002
Ponting                                           4 Toyota-0002
Bill                                              1 Audi-0003
8 rows selected.
SQL>Regards
Girish Sharma