How can set unique key checking by another table - sql

Question

i have create three tables: supplier, item and purchase. supplier id has relation with item table and item id has relation with purchase table. I do not want to insert itemid on purchase table in same supplier item. How can I set constraint?
CREATE TABLE csupplier(
supid NUMBER(10) PRIMARY KEY ,
supname VARCHAR2(30)
);
CREATE TABLE ctitem(
itemid NUMBER(10) PRIMARY KEY,
itemname VARCHAR2(50),
supid NUMBER(10)
);
ALTER TABLE CTITEM
ADD CONSTRAINT CTITEM_FK1 FOREIGN KEY(SUPID )REFERENCES CSUPPLIER(SUPID );
CREATE TABLE cPurchase(
purchaseid NUMBER(10) PRIMARY KEY,
itemid NUMBER(10),
purchaseqty NUMBER(10)
);
ALTER TABLE CPURCHASE
ADD CONSTRAINT CPURCHASE_FK1 FOREIGN KEY(ITEMID )REFERENCES CTITEM(ITEMID )

Answer 1

i don not want insert item-1 and item-3 in a same time under purchase
The problem is Oracle does not understand concept of at the same time. It understands transactions, it understands DML statements, it understands unique keys. So we need to frame your question in terms Oracle can understand: for instance, a given purchase cannot have more than one item from the same supplier.
Your first problem is that your data model can't support such a rule. Your cpurchase table has a primary key of purchaseid which means you have one record per item purchased. There is no set of purchased items against which we can enforce a rule. So, the first thing is to change the data model:
CREATE TABLE cPurchase(
purchaseid NUMBER(10) PRIMARY KEY );
CREATE TABLE cPurchaseItem(
purchaseid NUMBER(10),
itemid NUMBER(10),
purchaseqty NUMBER(10)
);
ALTER TABLE CPURCHASEITEM
ADD CONSTRAINT CPURCHASEITEM_PK PRIMARY KEY(PURCHASEID,ITEMID);
ALTER TABLE CPURCHASEITEM
ADD CONSTRAINT CPURCHASEITEM_FK1 FOREIGN KEY(PURCHASEID )REFERENCES CPURCHASE;
ALTER TABLE CPURCHASE
ADD CONSTRAINT CPURCHASE_FK2 FOREIGN KEY(ITEMID )REFERENCES CTITEM(ITEMID );
Now we have a header-detail structure which assigns multiple items to one purchase, which means we can attempt to enforce the rule.
The next problem is that supplierid is not an attribute of cpurchaseitem. There is no way to build a check constraint on a table or column which executes a query on another table. What you are after is a SQL Assertion, which is a notional construct that would allow us to define such rules. Alas Oracle (nor any other RDBMS) supports Assertions at the moment.
So that leaves us with three options:
Go procedural, and write a transaction API which enforces this rule.
Denormalise cpurchaeitem to include supplierid then build a unique constraint on (purchaseid, supplierid). You would need to populate supplierid whenever you populate cpurchaseitem.
Write an after statement trigger:
(Warning: this is coded wildstyle and may contain bugs and/or compilation errors.)
create or replace trigger cpurchaseitem_trg
after insert or update on cpurchaseitem
declare
rec_count number;
begin
select count(*)
into rec_count
from cpurchaseitem pi
join citem I on pi.itemid = i.itemid
group by pi.purchaseid, i.supplierid having count(*) > 1;
if rec_count > 0 then
raise_application_error(-20000
, 'more than one item for a supplier!');
end if;
end;
Frankly none of these solutions is especially appealing. The API is a solid solution but open to circumvention. The trigger will suffer from scaling issues as the number of purchases grows over time (although this can be mitigated by writing a compound trigger instead, left as an exercise for the reader). Denormalisation is the safest (and probably most performative) solution, even though it's not modelling best practice.

Answer 2

There are 2 solutions to your problem:
1. Alter the table cPurchase and add the supid column in the table. and make the unique key on this column. This will solve your problem.
CREATE TABLE cPurchase(
purchaseid NUMBER(10) PRIMARY KEY,
itemid NUMBER(10),
purchaseqty NUMBER(10),
supid NUMBER(10) UNIQUE KEY
);
If alter is not possible on this table, write a row level, Before Insert/update trigger. In this trigger write the logic to find the Supid based on the Item_id ctitem and them find any item on this supplier exists in your purchase table.
CREATE [ OR REPLACE ] TRIGGER SUP_CHECK
BEFORE INSERT
ON cPurchase
FOR EACH ROW
DECLARE
L_COUNT NUMBER;
BEGIN
SELECT COUNT(*) INTO L_COUNT
FROM cPurchase c
WHERE C.itemid in (Select itemid from ctitem ct where ct.supid = (Select supid
from ctitem where itemid = :new.itemid) );
EXCEPTION
WHEN ...
-- exception handling
END;