sql server foreign key

1-- On Create
2CREATE TABLE tableName (
3    ID INT,
4    SomeEntityID INT,
5    PRIMARY KEY (ID),
6    FOREIGN KEY (SomeEntityID)
7        REFERENCES SomeEntityTable(ID)
8        ON DELETE CASCADE
9);
10
11-- On Alter, if the column already exists but has no FK
12ALTER TABLE
13  tableName
14ADD
15  FOREIGN KEY (SomeEntityID) REFERENCES SomeEntityTable(ID) ON DELETE CASCADE;
16  
17 -- Add FK with a specific name
18 -- On Alter, if the column already exists but has no FK
19ALTER TABLE
20  tableName
21ADD CONSTRAINT fk_name
22  FOREIGN KEY (SomeEntityID) REFERENCES SomeEntityTable(ID) ON DELETE CASCADE;

1A FOREIGN KEY is a key used to link two tables together.
2A FOREIGN KEY is a field (or collection of fields) in one table that refers to the PRIMARY KEY in another table.
3The table containing the foreign key is called the child table, and the table containing the candidate key is called the referenced or parent table.
4
5Example:
6# creating table users
7CREATE TABLE users(
8	user_id INT NOT NULL,
9  	user_name VARCHAR(64) NOT NULL,
10  	user_pass VARCHAR(32) NOT NULL,
11  	PRIMARY KEY(user_id);
12);
13# adding user data
14INSERT INTO users VALUES(1,"Raj","raj@123");
15
16# creating table orders
17CREATE TABLE orders(
18	order_id INT NOT NULL,
19  	order_description VARCHAR(255),
20  	orderer_id INT NOT NULL,
21  	PRIMARY KEY(order_id),
22  	FOREIGN KEY (orderer_id) REFERENCES users(user_id)
23);
24# adding order data
25INSERT INTO orders VALUES(1,"Daily groceries",1);

1# A foreign key is essentially a reference to a primary
2# key in another table.
3
4# A Simple table of Users, 
5CREATE TABLE users(
6	userId INT NOT NULL,
7  	username VARCHAR(64) NOT NULL,
8  	passwd VARCHAR(32) NOT NULL,
9  	PRIMARY KEY(userId);
10);
11# Lets add a LEGIT user!
12INSERT INTO users VALUES(1000,"Terry","Teabagface$2");
13
14# We will create an order table that holds a reference
15# to an order made by our Terry
16CREATE TABLE orders(
17	orderId INT NOT NULL,
18  	orderDescription VARCHAR(255),
19  	ordererId INT NOT NULL,
20  	PRIMARY KEY(orderId),
21  	FOREIGN KEY (ordererId) REFERENCES users(userId)
22);
23# Now we can add an order from Terry
24INSERT INTO orders VALUES(0001,"Goat p0rn Weekly",1000);
25
26# Want to know more about the plight of Goats?
27# See the link below

1ALTER TABLE database.table
2  ADD COLUMN columnname INT DEFAULT(1),
3  ADD FOREIGN KEY fk_name(fk_column) REFERENCES reftable(refcolumn) ON DELETE CASCADE;

1CREATE TABLE Orders (
2    OrderID int NOT NULL PRIMARY KEY,
3    OrderNumber int NOT NULL,
4    PersonID int FOREIGN KEY REFERENCES Persons(PersonID)
5);

1CREATE TABLE Orders (
2    OrderID int NOT NULL,
3    OrderNumber int NOT NULL,
4    PersonID int,
5    PRIMARY KEY (OrderID),
6    FOREIGN KEY (PersonID) REFERENCES Persons(PersonID)
7);