SQL: Need help creating a one-to-many Set comparing query

Question

The parameters are thus:

1. I have a table called Tasks with columns ID (primary key) and label (text).
2. I have another table called Locations with a foreign key referencing Tasks' ID and a name for a location (text).
3. In my code I have a set of locations.
4. UPDATED: I need a query to return all tasks that have associated locations found within my set. Each task that has an associated location not found in my set must be thrown out.

What's the best way to go about this?

Answer 1

select * from tasks where id in (select unique task_id from locations where name in ('London', 'Geneva'...))

Don't forget to add an index on locations(name)

Answer 2

For the modified requirement:

select * from tasks t
where exists (select null from locations l
              where t.id = l.task_id and l.name in ('London', 'Geneva'...)) and
  not exists (select null from locations l
              where t.id = l.task_id and l.name not in ('London', 'Geneva'...))