What is the 2nd normal form? [duplicate]

Question

My informal representation of these are:

1NF: The table is divided so that no item will appear more than once.

2NF: I need a clear definition

3NF: Values can only be determined by the primary key.

I cannot make sense of it from the excerpts I found online or in my book. How do I differentiate between 1NF and 2NF?

Answer 1

Wikipedia says:

A table is in 2NF if and only if, it is in 1NF and every non-prime attribute of the table is either dependent on the whole of a candidate key, or on another non prime attribute.

To illustrate concept, let's use a table for an inventory of toys adapted from Head First SQL:

TOY_ID| STORE_ID| INVENTORY| STORE_ADDRESS

The primary key is composed of the attributes TOY_ID and STORE_ID. If we analyze the non-prime attribute INVENTORY we see that int depends on TOY_ID and STORE_ID at the same time. That's cool.

On the other hand, the non-prime attribute STORE_ADDRESS only depends on the attribute STORE_ID (i.e it's not related to the primary key attribute TOY_ID). That's a clear violation of 2NF, so to comply to with 2NF our schema must be like this:

An Inventory table: TOY_ID| STORE_ID| INVENTORY

and a Store table: STORE_ID| STORE_ADDRESS

Answer 2

A relation schema is in 2NF if every non-prime attribute is fully functionally dependent on every key.