A primary key is a column — or a group of columns — in a table that uniquely identifies the rows of data in that table. For example, in the table below, CustomerNo, which displays the ID number assigned to different customers, is the primary key.

The PRIMARY KEY constraint uniquely identifies each record in a table. Primary keys must contain UNIQUE values, and cannot contain NULL values. A table can have only ONE primary key; and in the table, this primary key can consist of single or multiple columns (fields).

Each record in a table is uniquely identified by the PRIMARY KEY constraint. Primary keys must have UNIQUE values and may not have NULL values. A table can only have ONE primary key, which can be made up of a single or several columns (fields).