A similar result can be accomplished in a relational database by what is called a join table. Either design choice creates redundant data in some form or another. And for especially large database needs, this could be very costly in terms of data storage and efficient data searching. Relational databases can give you more flexibility when it comes to data storage and those frequently asked questions. Relational databases do have their price, however.
Relational algebra is primary on the list.
There are very formal rules for categorizing data. The process of applying these rules is called normalization. Relations or tables rely on something called a key for identifying data sets. An attribute is a data item that tells you something about the key. The key not only tells you what you are dealing with, but also identifies how you search for specific records.