When people call them "SQL Databases", it's often a sign that they don't understand the relational model. Of course, it's much harder to make a RDBMS perform well if they don't, just as code written without an understanding of algorithms is likely to be unnecessarily slow.

Agreed. (Prepare to be downvoted for making a rational statement contrary to the prevailing fads.)

FYI: This isn't Reddit, and any meta-commentary about voting here is, at best, ignored as noise that detracts from conversation.

ObRelational: For people who want to learn about the relational model, two of the better presentations I've read are:

1. The Definitive Guide to SQLite by Mike Owens -- short, non-academic, theoretical enough to be useful. Focuses on SQLite, which is nice, because that way you don't take MySQL's flaws for granted.

2. An Introduction to Database Systems by C. J. Date -- academically thorough; it's a textbook, so the previous edition is pennies on the dollar. Mostly covers the relational model itself, rather than SQL, which is somewhat like comparing the pure untyped lambda calculus to Scheme. Once you understand the big ideas, though, everything else is just syntax and performance tuning (which are often implementation-specific anyway).