Can you explain an example of second normal form?

Question

Database design

Hello.
https://habr.com/post/113260/
I found on the Internet an example of a table with the following fields:
Detail, Warehouse, Quantity, Warehouse address.
And an explanation of the second normal form:

The key in this table is a pair of Item and Warehouse.
If there are no parts in the warehouse yet, then we do not have the opportunity to store the address of the warehouse, since the scheme does not provide for such a situation. Therefore, adding a new warehouse is impossible (insertion anomaly), and removing parts from the warehouse means that we will lose information about its address (deletion anomaly).

I have a question. Why can't we store the warehouse address if there are no parts on it? You can also specify an empty field in the Detail, and 0 in the Quantity.

Answer 1

The key in this table is a pair of Item and Warehouse.

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