Microsoft® SQL Server 2012 Unleashed

The PRIMARY KEY Constraint

The PRIMARY KEY constraint is one of the key methods for ensuring entity integrity. When this constraint is defined on a table, it ensures that every row can be uniquely identified with the primary key value(s). The primary key can have one or more columns as part of its definition. None of the columns in the primary key definition are allowed to have null values. When multiple columns are used in the definition of the primary key, the combination of the values in all the primary key columns must be unique. Duplication can exist in a single column that is part of a multicolumn primary key as long as the combination of other columns in the key is not duplicated in the tables.

There can be only one primary key defined ...