1. A primary key is a field in a table that contains unique data.
2. A Primary Key ist a non-empty set of columns that contain non-null values and whose values uniquely identify a row. A table cannot have more than one PrimaryKey. Often the term PrimaryKey is used to refer to a set of columns which is a strict subset of all of the columns in the table. In many cases it is assumed that the PrimaryKey consists of a single column.
_Databases_and_Networking/Databases/Primary_keys (1.); http://wiki.c2.com/?PrimaryKey (2.)