27. Difference between Primary Key and Foreign Key?

Primary Key	Foreign Key
Primary key uniquely identify a record in the table.	Foreign key is a field in the table that is primary key in another table.
Primary Key can't accept null values.	Foreign key can accept multiple null value.
By default, Primary key is clustered index and data in the database table is physically organized in the sequence of clustered index.	Foreign key do not automatically create an index, clustered or non-clustered. You can manually create an index on foreign key.
We can have only one Primary key in a table.	We can have more than one foreign key in a table.

Define Primary key and Foreign key

--Create Parent Table
CREATE TABLE Department
(
DeptID int PRIMARY KEY, --define primary key
Name varchar (50) NOT NULL,
Address varchar(100) NULL
)
GO
--Create Child Table
CREATE TABLE Employee
(
EmpID int PRIMARY KEY, --define primary key
Name varchar (50) NOT NULL,
Salary int NULL,
--define foreign key
DeptID int FOREIGN KEY REFERENCES Department(DeptID)
)