Define Primary and Unique key.

Amit Singh · 2010-12-13T00:00:00.000Z

Primary Key The column or columns of the table whose value uniquely identifies each row in the table is called primary key. You can define column as primary key using primary key constraint while you create table. When you define a column as primary key, a unique index is created which restricts duplicate data and fast access to data. A column defined as primary key doesn’t allow null value. By default, clustered index in created with the column having primary key. Unique key Unique key also ensures data uniqueness like primary key. A column with unique key defined allows null value. By default, it creates non-clustered index.

Amit Singh 3092 13 Dec 2010

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Updated 21 Sep 2020

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Amit Singh 13 Dec 2010

Primary Key

The column or columns of the table whose value uniquely identifies each row in the table is called primary key.
You can define column as primary key using primary key constraint while you create table.
When you define a column as primary key, a unique index is created which restricts duplicate data and fast access to data.
A column defined as primary key doesn’t allow null value.
By default, clustered index in created with the column having primary key.

Unique key

Unique key also ensures data uniqueness like primary key.
A column with unique key defined allows null value.
By default, it creates non-clustered index.

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Define Primary and Unique key.

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