How to compare two attributes from two different tables in a trigger? - database

Question

So I have two tables:
CREATE TABLE SALE(
OID_PA NUMBER(*,0) PRIMARY KEY,
Amount INTEGER NOT NULL,
Delivery_DATE DATE NOT NULL,
OID_V NUMBER(*,0) NOT NULL,
CONSTRAINT "AMOUNTCHECK" CHECK(Amount >=0),
FOREIGN KEY (OID_V) REFERENCES TICKET(OID_V));
and
CREATE TABLE PRODUCT
( Code NUMBER(*,0) PRIMARY KEY,
Stock INTEGER NOT NULL,
Price NUMBER(4,2) NOT NULL,
Production_Cost NUMBER NOT NULL,
Model VARCHAR2(50) NOT NULL,
TipoMueble VARCHAR2(50) NOT NULL,
TipoMaterial VARCHAR2(50) NOT NULL,
OID_PA NUMBER(*,0) NOT NULL,
CONSTRAINT "TipoMueble" CHECK(TipoMueble IN
('Canteado','Complementos','Glaciar',
'GrupoDiseño','Tiradores','Vitrinas')),
CONSTRAINT "TipoMaterial" CHECK(TipoMaterial IN ('Aluminio','Lacados',
'Polilaminado','Madera','Cristal','Chapa','Granito','Formica',
'Aglomerado','Marmol','Elementos de ferreteria')),
CONSTRAINT "StockNegativo" CHECK(Stock >=0),
CONSTRAINT "Precio" CHECK(Price>=0),
CONSTRAINT "CosteProduccion" CHECK (Production_cost>=0),
FOREIGN KEY (OID_PA) REFERENCES PAQUETE(OID_PA));
And now I want to create a trigger, the moment I modify the attribute "Amount" in the table "SALE" then it should check the table "PRODUCT".
if "stock" (in PRODUCT) is less than Amount (in SALE) then
RAISE_APPLICATION_ERROR(-20003, 'Not enough stock');
But I don't know how to make the trigger check in both tables at the same time.
right now my code looks likethis right now, it also has some errors that I'm not sure how to fix.
CREATE OR REPLACE TRIGGER TR_ENOUGH_STOCK
AFTER INSERT OR UPDATE OF Amount ON SALE
BEGIN
SELECT a.Amount, b.Stock
FROM ( SALE a inner join PRODUCT b on (a.Code = b.Code))
IF b.Stock < a.Amount THEN
RAISE APPLICATION ERROR(-20003, 'Not enough stock');
END IF;
END;
/
And I'm getting the following errors:
Error(209,5): PL/SQL: SQL Statement ignored
Error(212,5): PL/SQL: ORA-00933: SQL command not properly ended
Error(214,9): PLS-00103: Encountered the symbol "IF" when expecting one of
the following: ; <an identifier> <a double-quoted delimited-identifier>

Answer 1

Here's an example of how you might do that.
Create tables & a trigger:
SQL> create table sale (oid_pa number, amount number);
Table created.
SQL> create table product (code number, stock number, oid_pa number);
Table created.
SQL>
SQL> create or replace trigger trg_biu_stock
2 before insert or update
3 on sale
4 for each row
5 declare
6 l_stock product.stock%type;
7 begin
8 select stock into l_stock
9 from product
10 where oid_pa = :new.oid_pa;
11
12 if :new.amount > l_stock then
13 raise_application_error(-20000, 'Not enough stock');
14 end if;
15 end;
16 /
Trigger created.
SQL>
Insert sample records into the PRODUCT table:
SQL> insert into product values (1, 100, 10);
1 row created.
SQL> insert into product values (2, 50, 20);
1 row created.
SQL> select * From product;
CODE STOCK OID_PA
---------- ---------- ----------
1 100 10
2 50 20
SQL>
Let's test it:
SQL> -- 90 is less OID_PA = 10 stock value (which is 100)
SQL> insert into sale values (10, 90);
1 row created.
SQL> -- let's try to update it to a value larger than stock value:
SQL> update sale set amount = 200 where oid_pa = 10;
update sale set amount = 200 where oid_pa = 10
*
ERROR at line 1:
ORA-20000: Not enough stock
ORA-06512: at "HR.TRG_BIU_STOCK", line 9
ORA-04088: error during execution of trigger 'HR.TRG_BIU_STOCK'
SQL> -- how about a value which doesn't exceed the stock value?
SQL> update sale set amount = 5 where oid_pa = 10;
1 row updated.
SQL> select * From sale;
OID_PA AMOUNT
---------- ----------
10 5
SQL>
As of your attempt: apart from trigger code having wrong syntax, it wouldn't work anyway because you can't reference a table trigger is based on because it is right now being changed, i.e. it is mutating. Oracle raises an error in such cases (see an example below). The correct way is to reference that table's columns using :new or :old, as I did (see an example above).
SQL> create or replace trigger trg_biu_stock
2 before insert or update
3 on sale
4 for each row
5 declare
6 l_stock product.stock%type;
7 begin
8 select p.stock into l_stock
9 from product p
10 join sale s
11 on s.oid_pa = p.oid_pa
12 where p.oid_pa = :new.oid_pa;
13
14 if :new.amount > l_stock then
15 raise_application_error(-20000, 'Not enough stock');
16 end if;
17 end;
18 /
Trigger created.
SQL> update sale set amount = 50 where oid_pa = 10;
update sale set amount = 50 where oid_pa = 10
*
ERROR at line 1:
ORA-04091: table HR.SALE is mutating, trigger/function may not see it
ORA-06512: at "HR.TRG_BIU_STOCK", line 4
ORA-04088: error during execution of trigger 'HR.TRG_BIU_STOCK'
SQL>