While the Mathematica Entity system may serve the natural language interrogation facility, compared with any decent OO language structure it can be more than tedious to deal with, and "Enjoy" those "Strings" as "Keys" you have to remember and type everywhere instead of getters and setters!
It also seems to be limited in its ability to have one thing relate to another, and only seems to be intended for relatively shallow data structures.
And the docs and help pages are bit thin. For starters, if you want to find out how to actually use them you have to look under EntityStore[] and Set Up an Entity Store not Entity[].
There is a useful forum posting with a contribution from a Wolfram employee with some examples of how to find out the structure within a store.
To find out how to use the Entity system in combination with a database you may wish to start by reading this extensive tutorial Relational Databases Quick Start but it does not seem to explain how to create Database structures using the Entity systems, only how to query an existing database.