Consider we have data from visitors in a web site. And you are trying to create a database design for data mining.
There are some common fields which users have such as browser,ip, etc... . But there is a field such as email or something which for example 20% of people give. My question is at what percent you should merge tables and where we should move the email or custom field to another table?
in this case, is database size the only drawback of merging tables? and is it's performance results better than seperating table?