The Key benefit of Relational Databases is to maintain integrity. And they do this primarily via 2 concepts: (1)  Normalization: storing each fact in the db once  (2) Referential Integrity.<p>In an Invoices you have an Invoice Header, that should refer to multiple entities, buyer, seller, shipment address etc ... . Invoice detail that refer to the Header and represent a one to many relation between the data in the invoice header and the items in the invoice. Which may also have additional relationship attributes, like price, discount or any details specific to the item, for example, maybe your DB support a shipment address per item!<p>Anyway, in a relation DB an invoice refer to many entities (separate facts) and many relations (also separate facts) each worthy of its own table that refer to each other.<p>If you believe that Invoices are a good candidate for a document DB, you probably don't believe that the Relational Model is valid in general. Or that the 2 concepts I mentioned at first really help integrity!<p>The main flaw I see is that the relational model makes is hard to create dynamic models. A good Relational Model practice is that an entity in a Relational DB should represent an entity from your Universe Of Discourse (uod). That is the say, a model is better when tables represent real entities of the problem you are modelling! This is sometimes impossible when you want to store dynamic structures, some argue that this is not the fault of the Relational Model theory, but rather its implementations.