We have 2 Tables, lets call them: swd & swi

How does one generate a list that would say:
Show me all entries in swi added in last 30 days whom do not have their
id entered as matching key in swd?

We can generate the reverse of this list easy, that is show those ids in
swi that do have an entry in swd, via this:

FROM swd, swi
WHERE submited_date > (DATE_SUB(CURDATE(), INTERVAL 30 DAY))
AND approved_date IS NOT NULL
AND swi.id = swd.ix_id;

But how does one show the reverse?