primary-single, auto increment,not null,one primary key allowed unique-nullable,allowed unique key more,

Hi. in data level we have: Primary key = unique identifier of a row in DB & is NOT NULL. Unique key = unique identifier of a row in DB & is NULL ABLE. in perfect design model we have: Primary key = unique identifier of an entity and is meaningless. Unique key = unique identifier of a entity and is meaningful.

the primary diffrence bitween primary key & unique key is when we  assign primary key to a particular column cluster index automatically created for that column.

primary key= unique +not null+ clustered index

Both of them are same. we can say primary key=unique key+not null