# Primary Key
A Primary key is a [[Constraint]] that uniquely identifies [[SQL Database#Entity]] in a entity set (table). Only one pair of values that the primary key forms can exist within a entity set (table).

There can only be ONE primary per table and can consist of multiple attributes (columns).

A primary key is **not** required, but highly advised. Even if you don't use it its better to include one in your schema.

## Example

**Name (PK) - Age - Sex - Length**

In the table schema above, only one entity where the name is Nick can exist because Name is the primary key.