Why a table can have only one PK ?

Question

Just wanted to know the reason why Oracle allows only one PK per table ?
I came across a situation where my existing large PK which is on a single (ID) column needs an alteration to add a new column (No).
ID column on which PK is based is now subjected to have duplication and that is the reason why i want to add a new column. The new column composition is not going to ensure uniqueness.
So, i need to create (alter) PK on two columns now.
It seems, in order to do that, i need to drop the existing PK and create a new one. Since this is a OLTP production database, business cannot afford to drop the existing pk.
When i tried to create a separate PK on two columns, Oracle gave me below err:
ORA-02260: table can have only one primary keySo, i wanted to understand why Oracle allows only one PK / table ? and considering above err, is there any way to make required changes to my PK without altering existing PK?
(Table is a large 30 GB partition table and PK is a non partitioned global index)
I think PROBABLY i need to drop existing PK and re-create the new one. But core intention for opening this thread is to know rational behind one pk/table concept.

Answer 1

OraDBA02 wrote:
Just wanted to know the reason why Oracle allows only one PK per table ?
It's not exactly Oracle's fault. The ISO SQL standard requires that a "PRIMARY KEY" constraint can only be used once per table. However, a UNIQUE constraint on non-nullable columns achieves effectively the same thing as a PRIMARY KEY. You can have multiple UNIQUE constraints.
Arguably the PRIMARY KEY constraint is unnecessary and could safely be dropped from the SQL language. In principle in the relational model a "primary" key is nothing more or less than a candidate key. All keys are equal in the relational model and there is no fundamental reason to designate a primary key at all. There is certainly no reason not to have multiple keys. In the SQL world though it is a very widely observed convention to use a PRIMARY KEY constraint for each table and to use it for all foreign key references.