SELECT with large WHERE IN clause taking long time I'm pretty new at all this so be gentle :).

I have large medical DB where I need to extract data for patients based on selected doctor(s).

Right now I build a patient list by 

SELECT patID 
FROM appointments 
WHERE docID IN ('docid1', 'docid2', ..... , 'docidn') 


and once i get my patient list I use it when extracting data from a whole bunch of patient related tables. 

Depending on my initial doctors list I can end up with patient lists that are 20,000+. The total # of patients in the system is close to 1,000,000.

I then take my patient list and start getting data from other tables, more or less like this

Select * 
FROM xrays 
WHERE patID IN ('patid1', 'patid2', 'paid3',....., 'patidn' ) 


where I build the list of patids based on my the list I have from the first part.

As you can imagine, with a patient list of 20,000, the above query is taking a long time.

Any suggestions on a better way to do this?
This is what Joins are for. You don't have to get a list of patients ids and then send them back to the SQL engine. You can combine the two queries into one, with JOIN:

SELECT x.* 
FROM xrays AS x
  JOIN appointments AS a
    ON a.patID = x.patID 
WHERE a.docID IN ('docid1', 'docid2', ..... , 'docidn') 
ORDER BY x.patID                   --- optional so your application gets
                                   --- the data of patients, ordered.


Other things to check is the indexes you have on the tables. As a rule of thumb, at least all columns used in joins, like your docID, patID, should have an index.