title: Why we need a primary key?
body: <p>I am reading about primary keys and at a lot of tutorials, technical blogs etc., and I found that a primary key cannot be null. I think it's totally wrong because I was able to insert null value in the column.
  I suppose a primary key can have a not null value only if the column is declared as not null. But again this is not a feature of primary keys.</p>
  <p>My question is why do we have a concept of primary key because I find only one difference between primary key and unique key is that &quot;Primary key can be declared only on one column whereas unique key can be declared on multiple columns&quot;. So my understanding is that why can't we also declare the primary key as a unique key if we don't have any other difference.</p>