Answer Posted / vital reddy

suppose you have landline and through the landline you have
taken net connection .
so at the end of the month you will get two bills on the
landline which is subject to one means

primary key is landline which is parent one ( customer
code available)
broadband and landline ( two bills generation ) are the
unique keys called child keys .(contract ids available)
so in this scenario contract ids can be null but cannot be
duplicate .
One more You cannot remove the primary key until and unless
remove the branched ones.

I have good knowledge on GSM /broadband/pstn technologies .

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