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&quo