SQL Tutorials - 6.Constraints - Primary key / Unique Key / Foreign Key

In this tutorial we will learn how to create a keys and how they are useful for us

PRIMARY KEY constraint : uniquely identifies each record in a table.
Primary keys must contain UNIQUE values, and cannot contain NULL values.

The UNIQUE constraint : ensures that all values in a column are different and it's allow one NULL value.

FOREIGN KEY : is a key used along with Primary Key to link two tables together and the table with Foreign key called Child table and with Primary key called Parent table.

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