A Primary Key (PK) defines one or more columns that uniquely identify a row in a table. Because it is used for identification, no column in a PK can be null. If you change or reuse a PK, historical information and relationships with others tables may be lost.
PK is a data modeling term and logical relational database concept. The columns identified as the PK during the data modeling process are often chosen for the UPI of a table. However, physical database design considerations often lead to a choice of columns other than those of the primary key for the PI of a table.