Selecting primary keys that do not have foreign keys in another table

Question

For simplification, I have two tables related with one to many using a foreign key, for example:

Users table:
id
name

Actions table:
id
user_id

<

Answer 1

SELECT u.id
FROM users u
LEFT JOIN actions a
   ON a.user_id = u.id
WHERE a.user_id IS NULL

Answer 2

Optimized version would be:

SELECT u.id
FROM users u
LEFT JOIN actions a
ON a.user_id = u.id
AND ISNULL(a.user_id)

Answer 3

select u.id
from users u
left outer join actions a on a.user_id = u.id
where a.user_id is null