Selecting Rows From One Table With A Key That Doesn't Exist In Another.

I'm trying to select rows from one table that does not have a key in another table.

For instance:

I have a hospital table with two columns, id and name.

I have a patients table with three columns, id, name, and locationid.

Now I want to find those hospitals that have no patients. So I want to get those hospitals whos id does not equal any of the locationid's in the patients table.