From time to time I happen to interview BI developers and I noticed that many of them don't understand how joins work. Probably, because most of the time they used to work with normalized data in transactional systems where primary keys always exist and defined by database design. In order to do figure out if the candidate has basic understanding of joins I ask him/her to answer the question below (without executing any actual query):
Hint: the correct answer is not 4. If you're unsure whether your answer is correct see this short video where both tables are joined using EasyMorph: https://www.youtube.com/watch?v=RYCtoRTEk84, or check our this SQLFiddle: http://sqlfiddle.com/#!9/60011/11/0
Not understanding joins sooner or later leads to uncontrolled data duplication in cases where joined tables are denormalized, which is a frequent cause of miscalculations in analytical applications.
UPDATE: Added a link to SQLFiddle (kudos to Devon Guerro).