The PRIMARY KEY constraint in SQL Server ensures that each entry in a table is uniquely recognized.
Primary keys must have UNIQUE values and cannot have NULL values.
In SQL Server, a table can only have one primary key, which can be made up of one or more columns (fields).
What is the primary key in SQL Server?
In SQL Server, a Primary Key is a field or column which uniquely identifies each and every record in a table.
- A primary key column doesn’t allow to insert null or duplicate values.
- A Primary key can consist of up to 16 columns.
- A table can have only one primary key which can be applied on single or multiple columns.
- The primary key consist of multiple columns or fields is known as the Composite primary key.
- A Primary Key constraint can be applied to the columns having any data types.
- A primary key by default creates a clustered index on the column.
A primary key can be created at the column or table level.
Syntax to create a primary key
The following are the syntax to create a primary key in SQL server.