Question

I was looking for info on foreign keys…. AGAIN! … and happened to notice on webschools.com they have different examples of the same thing. for the foreign key example they have

CREATE TABLE Orders
(
O_Id int NOT NULL,
OrderNo int NOT NULL,
P_Id int,
PRIMARY KEY (O_Id),
FOREIGN KEY (P_Id) REFERENCES Persons(P_Id)
)

---

CREATE TABLE Orders
(
O_Id int NOT NULL PRIMARY KEY,
OrderNo int NOT NULL,
P_Id int FOREIGN KEY REFERENCES Persons(P_Id)
)

---

CREATE TABLE Orders
(
O_Id int NOT NULL,
OrderNo int NOT NULL,
P_Id int,
PRIMARY KEY (O_Id),
CONSTRAINT fk_PerOrders FOREIGN KEY (P_Id)
REFERENCES Persons(P_Id)
)

now……….

what’s the difference?…

How do I know which one I’m suppose to use for my database? I have a feeling this will help resolve a lot of the confusion I’m having with SQL…

Answer 1

No difference in effect: They achieve exactly the same thing.

I prefer the in-line version, because it puts the fk definition as close to the column definition as possible.

There’s a 3rd way – a separate alter table statement (which I think is the “official” way):

alter table orders
add contraint fk_PerOrders 
foreign key p_id references persons(p_id);

You may find some databases don’t support one version or the other.