Aaaahhhhh ... rails developers. I take your point.

As to CHAR vs VARCHAR -- well, is it fixed length or constantly changing? The clue is in the name. And CHAR is usually up to 50% faster on overhead and VARCHAR uses padding which can be a huge waste of space. They should just know this.

Then, SQL Joins are plain English and should be easily visualised in the abstract (it does help if you can see the data in front of you though, I can't deny).

Date vs Time is something I am finding confuses lots of people. Precision as well.