The existence of data in a Primary key field is:

The correct answer is option 3- Required and must be unique.

The existence of data in a Primary key field is required and must be unique. Relationship between tables is established with the help of primary key and foreign key. Primary key consists of minimum possible one or more than one attributes of a table, which uniquely identifies each row of that table. Foreign key consists of set of attributes, which from primary key in another (related) table.