primary key in livesql code example

Example 1: primary key in sql

PRIMARY KEY
   -- unique identifier for the entire row of record in a table
   --  can not be null and must be unique

Example 2: primary key in sql

The PRIMARY KEY constraint uniquely identifies each record in a table.
Primary keys must contain UNIQUE values, and cannot contain NULL values. 
A table can have only ONE primary key; and in the table, this primary key can consist of single or multiple columns (fields).