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I have come across a great example to explain this overlapping of candidate keys:

Let us suppose that in some system, a Customer has one CreditCard, and a CreditCard belongs to one Customer. In the Rentals table, you uniquely identify a Rental by the EquipmentID, Date and CustomerID.

The FDs be:

{CustomerId, EquipmentId, Date} -> {CreditCard}
{CustomerId} -> {CreditCard}

 

But since the association is one-to-one, the following FDs also hold:

{CreditCard} -> {CustomerId}
{CreditCard, EquipmentId, Date} -> {CustomerId}

Since CustomerId and CreditCard can be used interchangeably to uniquely identify your customer.

In the scenario above, you have overlapping candidate keys:

{CreditCard, EquipmentId, Date} {CustomerId, EquipmentId, Date}

They are overlapping because they are composite keys (they consist of more than one attribute) and because at least one of their attributes is shared (in this case, they share both EquipmentId and Date.

 

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Ans. (D)

Not in BCNF but in 3NF

Again MN NO both are key and having M in common

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