Primary Key Constraint In SQL

 Primary Key::constraint

======================

 ->It is combination of Unique and not null

->Restricted Duplicates and Nulls

->A table is having Only one primary Key

At column Level::
=================

EX::

SQL> create table test2(eid int primary key,ename varchar2(10) primary key);

create table test2(eid int primary key,ename varchar2(10) primary key)

                                                          *

ERROR at line 1:

ORA-02260: table can have only one primary key

SQL> create table test2(eid int primary key,ename varchar2(10));

Table created.

SQL> insert into test2 values(1,'sam');

1 row created.

SQL> insert into test2 values(2,'dan');

1 row created.

SQL> select * from test2;

       EID ENAME

---------- ----------

         1 sam

         2 dan

SQL> insert into test2 values(null,'dan');

insert into test2 values(null,'dan')

                         *

ERROR at line 1:

ORA-01400: cannot insert NULL into ("SYSTEM"."TEST2"."EID")

SQL> insert into test2 values(2,'raja');

insert into test2 values(2,'raja')

*

ERROR at line 1:

ORA-00001: unique constraint (SYSTEM.SYS_C007655) violated

SQL> insert into test2 values(3,'sam');

1 row created.

SQL> insert into test2 values(4,null);

1 row created.

SQL> select * from test2;

       EID ENAME

---------- ----------

         1 sam

         2 dan

         3 sam

         4

At table level::
==================

EX::

SQL> create table test2(eid int,ename varchar2(10),primary key(eid,ename));

Table created.

SQL> insert into test2 values(1,'sam');

1 row created.

SQL> insert into test2 values(2,'dan');

1 row created.

SQL> select * from test2;

       EID ENAME

---------- ----------

         1 sam

         2 dan

SQL> insert into test2 values(1,'jhon');

1 row created.

SQL> select * from test2;

       EID ENAME

---------- ----------

         1 sam

         2 dan

         1 jhon

SQL> insert into test2 values(1,'sam');

insert into test2 values(1,'sam')

*

ERROR at line 1:

ORA-00001: unique constraint (SYSTEM.SYS_C007654) violated

SQL> insert into test2 values(null,null);

insert into test2 values(null,null)

                         *

ERROR at line 1:

ORA-01400: cannot insert NULL into ("SYSTEM"."TEST2"."EID")

SQL> insert into test2 values(2,null);

insert into test2 values(2,null)

                           *

ERROR at line 1:

ORA-01400: cannot insert NULL into ("SYSTEM"."TEST2"."ENAME")

SQL> select * from test2;

       EID ENAME

---------- ----------

         1 sam

         2 dan

         1 jhon