If a relation contain composite or multi-valued attribute, it violates first normal form
A relation is in first normal form if every attribute in that relation is singled valued attribute.
Example
Practical Note : It would be trouble some to keep phone number or email id as a different entries and also would be tempting too to put email ids and phone numbers in different table but suppose a requirement comes up where you have to black list email ids in your entire product or you have to black list set of phone numbers then it would be a very huge task to handle it.
The important question here you need to ask the requirement is is the phone number or email id’s a valued attribute i.e is there a possibility that any change will occur in future in respect to that attribute like back listing some email ids or phone numbers or blacklisting some domains or county code.
And if yes the email id or phone number is a valued attribute then you need to add field (column in DB) is_enabled so that you can disable to enabled these email ids or mobile numbers at anytime.
