Foreign Key

A foreign Key is a combination of columns with value is based on the primary key values from another table. A foreign key constraint also known as Referential Integrity Constraint

A foreign key is an attribute of the table which have the reference of a primary key in Second Table.

CREATE TABLE Orders
(
O_Id int NOT NULL PRIMARY KEY,
OrderNo int NOT NULL,
P_Id int FOREIGN KEY REFERENCES Persons(P_Id)
)

and Here on question arise that is, foreign key should be primary key in second table or not?

 So don't be confuse i am clearing you.

Its not necessary that foreign key should be unique or primary key.

it is simple an attribute as other attribute in table.

Note-if you are very confused so do it practically. make table and apply all keys and try to understand.

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