the steps in a left join 
even more than usual these steps aren't *actually* what happens (it would be way too slow) but this is an accurate way to understand what results the join will return
(what actually usually happens is that you'll do the join on a column that's indexed so it'll be able to figure out all the matches much faster by using the index)
a problem with this explanation is that you _can_ join on an arbitrary boolean condition ((a = b or c = d) and e <= f +2) but honestly I never do that and find it super confusing to think about, I always just do JOIN ON table1.something = table2.something_else
