what are foreign keys code example

Example 1: add constraint fk

ALTER TABLE Orders
ADD CONSTRAINT FK_PersonOrder
FOREIGN KEY (PersonID) REFERENCES Persons(PersonID);

Example 2: foreign key

Foreign Key: 
It is a column that comes from a different table and
using Foreign key tables are related each other
It is the primary key of another table
It can be duplicate or null for another table


Primary Key :
It is unique column in every table in a database
It can ONLY accept;
    - nonduplicate values
    - cannot be NULL

Unique Key:
Only unique value and also can contain NULL

Example 3: What is foreign key?

Foreign Key is a non-key attribute which is derived from the primary key 
of another table which links those tables together.

Example 4: what is foreign key

Foreign Key: 
It is a column that comes from a different table and
using Foreign key tables are related each other
It is the primary key of another table
It can be duplicate or null for another table