Unique Key

Unique Key : – Unique Key does not allow duplicate value but it accept NULL value many times (As every null is unique and never equal to another null).

1) Unique Key (Column Level) :-

a) With System defined name

SQL>create table student(regno number(10) unique, name varchar2(10), dob date, course varchar2(10));

b) With User defined name

SQL>create table student(regno number(10) constraint uk_regno unique,

                     name varchar2(10),dob date,course varchar2(10));

2) Unique Key (Table Level) :-

a) With System defined name

SQL>create table student(regno number(10),name varchar2(10),dob date,course varchar2(10), unique(regno));

b) With User defined name

SQL>create table student(regno number(10),name varchar2(10),dob date,course varchar2(10),constraint uk_regno unique(regno));

3) Composite Unique Key :- Unique Key created for more than one column is called Composite Unique Key.

a) With System defined name

SQL>create table student(regno number(10),name varchar2(10),dob date,course varchar2(10), unique(regno,name));

b) With User defined name

SQL>create table student(regno number(10),name varchar2(10),dob date,course varchar2(10),constraint uk_regno unique(regno,name));                  

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