JOINs, right? They can be a little tough to think about, especially when you're just starting out with SQL. Jeff Atwood's Venn diagram visualization of how JOINs work - you know, what goes in a LEFT JOIN but not a RIGHT JOIN, that kind of thing - is fairly well-established, but for a quick and interactive variation on the same idea, try this alternative from Patrick Spathon. You have your standard Venn diagrams:
But each one can be selected to display a table (along with its null values), the table to JOIN to, and the combined results of the JOIN. It's a great way to learn the difference, or to quickly reference when it's three in the morning and things are just starting to not make so much sense.