Q.What is a primary key? Why must its value be unique and not left blank?

The primary key is the attribute that identifies each tuple uniquely; this question asks what it is and why its two rules matter.

A primary key is an attribute (or a combination of attributes) whose value uniquely identifies each tuple in a relation. For example, in a Student table, RollNo is a natural primary key because every student has a different roll number.

Why the value must be unique: if two tuples had the same primary-key value, there would be no way to tell them apart — asking for 'the student with RollNo 5' would return two different students. Uniqueness guarantees that each record can be picked out with no confusion.

Why it must never be left blank (NULL): the whole job of the primary key is to identify a record. If it were left empty, that record could not be reliably identified or referred to by other tables (through a foreign key). So a primary key must always contain a value. …

A primary key need not be a 'number' — it just has to be unique and non-blank. Also, do not confuse a primary key (identifies a record withi …