What is the primary key?

A. A table that doesn't relate to any other tables

B. The one field in the table that contains only unique entries

C. The duplicate field in a table that crosses multiple tables

D. An object providing determining data on the properties for each field

Only B is true, but it suggests there is only one such field in a table. You can mark multiple fields as unique. A primary key is intended to uniquely identify a table record (e. g., from other tables) and in order to do that, it has to be unique (ie., this is a consequence of it being a primary key).

A primary key is a special relational database table column (or combination of columns) designated to uniquely identify all table records. A primary key's main features are: It must contain a unique value for each row of data. It cannot contain null values.