abinay

• Oct 6th, 2012

 

the combination of unique key and not null key is called primary key.
In a table we use only one primary key.It is unique,it cant allow the null values.

UNIQUE key ,in a table we use more than one unique key.It is also a unique number,but it allows null values.

If a table having the more than one unique columns,in that situation we use one column as primary key, remain once use unique key.