UNIQUE

This is used to avoid duplicates but it allow nulls.

We can add this constraint in all three levels.

Ex:

      COLUMN LEVEL

     SQL> create table student12(no number(2) unique, name varchar(10), marks

             number(3));

      SQL> create table student12(no number(2)  constraint un unique, name varchar(10),

             marks number(3));

       TABLE LEVEL

     

      SQL> create table student12(no number(2) , name varchar(10), marks number(3),

             unique(no));

      SQL> create table student12(no number(2) , name varchar(10), marks number(3),

             constraint un unique(no));

      ALTER LEVEL

      SQL> alter table student12 add unique(no);

      SQL> alter table student12 add constraint un unique(no);