Off the top of my head, I&#x27;d say the kernels of database systems are the CAP on the operational side and ACID on the design side. More abstractly, what really matters is the <i>particular</i> business logic [what problem are we trying to solve?] a database models not the implementation details [what code are we using?].<p>That said, the bigger abstraction is information storage and retrieval. For some purposes a printed book on a shelf is most appropriate, and it&#x27;s helpful to think of the continuum of storage technologies as including hard copy simply because it filters out the noise of implementations: key-value stores, versus document stores, versus relational stores, versus column stores, versus CPU caches. The what and why and when come before the how and where.<p>For traditional relational databases, I found <i>Database Systems: The Complete Book</i> very informative. But my intuition is that RDMS&#x27;s are increasingly less likely to be the best choice for a lot of common applications.<p>Good luck.