A primary key is a field or a combination of fields (columns, to be accurate) that uniquely identifies each record (or row)in a table. It is common to use social security numbers as primary keys because it's obvious that every person has a unique social security number. If there is no appropriate field you can choose as a primary key, you can use system-generated numbers to uniquely identify your rows. A primary key must be unique and present in every row of the table to maintain the validity of the data.
