what is the purpose of a foreign key in a database code example

Example 1: 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


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 2: 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.

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Sql Example

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