SQL & PL/SQL

i am trying to create Unique Constraint of table oms_product_group's Column product_group_name.

it's parent child relationship like parent_id will be same for some the product_group_name

Example:

product_group_name         parent_id

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

All                                           0

Mobile                                     1

Laptop                                    1

Men                                        2  

Men                                        2 (<--- if i have already created MEN then it must not allow me create it again)

Android                                   3

What i have tried.

alter table OMS_PRODUCT_GROUP add constraint OMS_PRODUCT_GROUP_NAME_UN unique (PRODUCT_GROUP_NAME) DEFERRABLE INITIALLY IMMEDIATE;

ERROR: ORA-02299: cannot validate (ECUBE.OMS_PRODUCT_GROUP_NAME_UN) - duplicate keys found

alter table OMS_PRODUCT_GROUP add novalidate constraint OMS_PRODUCT_GROUP_NAME_UN

      using index (create index unique_index on OMS_PRODUCT_GROUP(PRODUCT_GROUP_NAME));

ORA-02253: constraint specification not allowed here

So basically if some one inserted one value name he/she can not insert same value name again that's i am creating Unique Key Constraints.

Thank You

Pranav Shah.