The two requirements of first normal form:
The domain of an attribute must include only atomic values.
The value of any attribute in a tuple must be a single value from the domain of that attribute.
1NF does not allow multi-valued attributes, composite attributes or combinations thereof. This means no single value for a single tuple can be a list (eg. a person's children). There can also not be any nested relations (eg. an address attribute, which itself has sub-attributes of number, street name and city).
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