Valid refutation. You can use an RDBMS without making use of most of its facilities, and indeed, that is the typical usage case.
You can easily have a non-trivial app which makes what you'd probably consider trivial usage of a database.
> someone claiming knowledge of an RDBMS and not knowing how to do a simple join across two tables is like someone claiming to be an expert in c and not knowing how to write a for loop (let alone use pointers)..
You're mixing up "working knowledge of SQL" with "expert knowledge of RDBMSes". I agree I am nowhere close to an RDBMS expert. However, I can use SQL, which was the point of the original post.
> after claiming to have worked with RDBMS for ages
Something is wrong in your mental model of my description. I used MySQL for years, starting many years ago. I did not say I was an expert in RDBMSes, or used advanced functionality in any sense, and I said the join functionality was done at the application level. My usage of SQL was basic, but I have a working knowledge of it. If there's something I need but don't know, I can read the docs and do it.
> it would take what, one day of reading a basic book on SQL to understand what a join is and formulate a query?
Reading an SQL book doesn't mean you're going to recall the syntax, until you have written queries a number of times by hand. I agree it is not complicated. I do not agree that asking someone to write a JOIN tells you anything about WHY they can't do it.
> The danger of NOT using a quick and easy litmus test is that all kinds of idiots get through to your personal interview stage
You'd probably filter out Pete Norvig, and Linus, and who knows how many other expert programmers, if you asked them to write an SQL join off the top of their head.