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2nd normal key to understanding a candidate

To understand what is the second normal form, I read some articles, and there are some things that I do not understand.

The article here in the client table says that it is not in 2NF because there are several attributes which don’t completely rely on the entire Customer table primary key. Here, by primary key, I think it meant {customerId, EmployeeId}

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{customerId, employeeId} -, , , customerCity, PostalCode - , , 2NF. , - , Customer customerId ? ( employeeId customerId).
CustomerId -, {CustomerId, EmployeeId} -, - - .

, customerId, , 2NF?
, 2NF , .
To indicate which customers are called upon by each employee To identify customers and their locations.
, 2NF.
, ?

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3NF. TABLE_BOOK - bookId? {bookId, genereId} ? , 2NF, genreId.

-, , .

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. , -. -, . - , , , , . , , .

, CustomerID Customer, 2NF. EmployeeID - : CustomerID- > EmployeeID. , , , , EmployeeID . , , , {CustomerID, EmployeeID} , CustomerID.

. BookID , , BookID, GenreID : BookID- > GenreID, BookID- > Price. {BookID, GenreID} , BookID- > GenreID, BookID- > Price , . 2NF BookID- > Price.

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