SQL primary key, foreign key

PRIMARY KEY 是为了保证数据的完整性

FOREIGN KEY 是为了保证数据引用的完整性

The PRIMARY KEY constraint uniquely identifies each record in a database table.

CREATE TABLE Persons
(
Id_P int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
[code]PRIMARY KEY (Id_P)

)[/code]

A FOREIGN KEY in one table points to a PRIMARY KEY in another table.

Example:
The "Persons" table:

P_Id	LastName	FirstName	Address	City
1	Hansen	Ola	Timoteivn 10	Sandnes
2	Svendson	Tove	Borgvn 23	Sandnes
3	Pettersen	Kari	Storgt 20	Stavanger

The "Orders" table:

O_Id	OrderNo	P_Id
1	77895	3
2	44678	3
3	22456	2
4	24562	1

The "P_Id" column in the "Persons" table is the PRIMARY KEY in the "Persons" table.
The "P_Id" column in the "Orders" table is a FOREIGN KEY in the "Orders" table.

用法:

The following SQL creates a FOREIGN KEY on the "P_Id" column when the "Orders" table is created:

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)
)

即要求foreign key必须是person中的primary key,否则这一条目不成立.