I got a little ahead of myself. First, I should explain the interesting little creature you see on screen.
This is the Sequel dataset, and it is what gives Sequel a lot of its flexibility.
A dataset represents an SQL query, or more generally, an abstract set of objects.
At any point you can turn that abstract set into a concrete set by calling all.
As shown here, datasets are usually created by calling the array access operator on the database object with a symbol.