However, what if we want to further filter the rows to only include characters whose father's name is Bill?
In this case, we need to join the characters table to itself to find the father for each character, and only return rows where the father is Bill.
Let me briefly sidetrack and state you can use a triple underscore in a symbol to specify an alias, which is how the characters table is aliased to father when joining.
However, this query won't work because the database won't know if the unqualified columns in the conditions apply to the main character or to the character's father.