Unique value

Hi friends,

I have a tables A and B

DESC A

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

ID NUMBER

NAME VARHAR2(10)

DESC B

-----------

ID NUMBER

SUBNAME VARCHAR2(20);

In the above table B.ID is the foreign key to A.ID.

How can I create unique value for B.SUBNAME based on A.NAME.

Means , A.NAME has value 'AAAA" and
B.SUBNAME has values 'BBBB', 'CCCC';

when I insert value again in B.SUBNAME as 'BBBB' it should not agree for the combination value of A.NAME('AAAA'),

but 'BBBB' can agree when I enter value for A.NAME other than 'AAAA'

Please give some ideas,

Thanks in advance

syed.

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