Double query

Simple question - but my brain is not working:

This is my table
Column   |          Type          |                            
Modifiers                           
-----------+------------------------+-----------------------------------------------------------------id        |
integer               | not null default 
 
nextval('emr_clinicschedule_id_seq'::regclass)clinic_id | integer                | not nulldate      | date
     |day       | smallint               |status    | boolean                | not nullopen      | time without time
zone|close     | time without time zone |reason    | character varying(64)  |active    | boolean                | not
null

I want to find any value:

SELECT id FROM schedule WHERE clinic_id = '%s' AND date = '%s'

But I want to make sure the clinic exist first then find if the date 
exist 2nd.

How do I do that?

Thanks in advance,
J