Help me write a trigger : I have no idea how to :(

Question

Hi,
I have to design a trigger based on the following requirement:
I have an ASSET table ASSET (asset_id number(10) (primary key) , site_id (foreign key) references SITE(site_id) , cost_ctr_id (foreign key) references COST_CENTER(cost_ctr_id) )
I have a COST_CENTER table COST_CENTER (cost_center_id number(10) primary key) I have a SITE table SITE (site_id number(10) primary key)
I have a SITE_COST_CTR table SITE_COST_CTR (site_id (foreign key) references SITE(site_id ) , cost_ctr_id (foreign key) references COST_CENTER(cost_ctr_id) ) here (site_id, cost_ctr_id) is the composite primary key .
Now I want a trigger which should satisfy the following : Whenever there is an insert or update in the ASSET table, it should first check in the SITE table and if the site_id is not there it should insert that site_id in the site table, similarly for cost_center_id it should see the cost center table and insert cost_center_id there, followed by an insert into SITE_COST_CTR table (It should insert site_id, cost_center_id in SITE_COST_CTR table)
Can anyone please help me with this trigger ? I have no idea, never ever done it till date :'(. Please help me.
Here's my effort so far, and I am not even a novice in this area!
CREATE OR REPLACE TRIGGER TR_SITE_COST_CTR
BEFORE INSERT OR UPDATE ON ASSET
REFERENCING NEW AS NEW_ASSET
FOR EACH ROW
WHEN ( (SELECT COUNT(*) FROM SITE WHERE SITE_ID = NEW_ASSET.SITE_ID) > 1 AND (SELECT COUNT(*) FROM COST_CENTER WHERE COST_CENTER_ID = NEW_ASSET.COST_CENTER_ID) > 1 )
BEGIN
INSERT INTO SITE (SITE_ID) VALUES (:NEW_ASSET.SITE_ID);
INSERT INTO COST_CENTER VALUES (:NEW_ASSET.COST_CENTER_ID , :NEW_ASSET.COST_CENTER_ID); INSERT INTO SITE_COST_CENTER VALUES (:NEW_ASSET.SITE_ID, :NEW_ASSET.COST_CENTER_ID);
END
Thanks in advance :)

Answer 1

SQL> create table site (site_id number(10) primary key)
  2  /
Tabel is aangemaakt.
SQL> create table cost_center (cost_ctr_id number(10) primary key)
  2  /
Tabel is aangemaakt.
SQL> create table site_cost_ctr
  2  ( site_id     number(10) references site(site_id)
  3  , cost_ctr_id number(10) references cost_center(cost_ctr_id)
  4  , primary key (site_id,cost_ctr_id)
  5  )
  6  /
Tabel is aangemaakt.
SQL> create table asset
  2  ( asset_id    number(10) primary key
  3  , site_id     number(10) references site(site_id)
  4  , cost_ctr_id number(10) references cost_center(cost_ctr_id)
  5  )
  6  /
Tabel is aangemaakt.
SQL> create trigger tr_site_cost_ctr
  2  before insert or update on asset for each row
  3  declare
  4    l_dummy number;
  5  begin
  6    begin
  7      select site_id
  8        into l_dummy
  9        from site
10       where site_id = :new.site_id
11      ;
12    exception
13    when no_data_found then
14      insert into site (site_id) values (:new.site_id);
15    end
16    ;
17    begin
18      select cost_ctr_id
19        into l_dummy
20        from cost_center
21       where cost_ctr_id = :new.cost_ctr_id
22      ;
23    exception
24    when no_data_found then
25      insert into cost_center (cost_ctr_id) values (:new.cost_ctr_id);
26    end
27    ;
28    begin
29      select site_id
30        into l_dummy
31        from site_cost_ctr
32       where site_id = :new.site_id
33         and cost_ctr_id = :new.cost_ctr_id
34      ;
35    exception
36    when no_data_found then
37      insert into site_cost_ctr (site_id,cost_ctr_id) values (:new.site_id,:new.cost_ctr_id);
38    end
39    ;
40  end;
41  /
Trigger is aangemaakt.
SQL> insert into asset values (1, 11, 21)
  2  /
1 rij is aangemaakt.
SQL> insert into asset values (2, 11, 22)
  2  /
1 rij is aangemaakt.
SQL> insert into asset values (3, 12, 21)
  2  /
1 rij is aangemaakt.
SQL> insert into asset values (4, 11, 21)
  2  /
1 rij is aangemaakt.
SQL> select * from site
  2  /
                               SITE_ID
                                    11
                                    12
2 rijen zijn geselecteerd.
SQL> select * from cost_center
  2  /
                           COST_CTR_ID
                                    21
                                    22
2 rijen zijn geselecteerd.
SQL> select * from site_cost_ctr
  2  /
                               SITE_ID                            COST_CTR_ID
                                    11                                     21
                                    11                                     22
                                    12                                     21
3 rijen zijn geselecteerd.
SQL> select * from asset
  2  /
                              ASSET_ID                                SITE_ID                            COST_CTR_ID
                                     1                                     11                                 21
                                     2                                     11                                 22
                                     3                                     12                                 21
                                     4                                     11                                 21
4 rijen zijn geselecteerd.Regards,
Rob.