Query extensibility with WHERE EXISTS with a large table

Question

The following query is designed to find the number of people who went to a hospital, the total number of people who went to a hospital and the divide those two to find a percentage. The table Claims is two million plus rows and does have the correct non-clustered index of patientid, admissiondate, and dischargdate. The query runs quickly enough but I'm interested in how I could make it more usable. I would like to be able to add another code in the line where (hcpcs.hcpcs ='97001') and have the change in percentRehabNotHomeHealth be relfected in another column. Is there possible without writing a big, fat join statement where I join the results of the two queries together? I know that by adding the extra column the math won't look right, but I'm not worried about that at the moment. desired sample output: http://imgur.com/BCLrd
database schema

select  h.hospitalname
    ,count(*) as visitCounts
    ,hospitalcounts
    ,round(count(*)/cast(hospitalcounts as float) *100,2) as percentRehabNotHomeHealth
    from Patient p
    inner join statecounties as sc on sc.countycode = p.countycode
    and sc.statecode = p.statecode
    inner join hospitals as h on h.npi=p.hospitalnpi
    inner join
    --this join adds the hospitalCounts column
    (
        select h.hospitalname, count(*) as hospitalCounts
            from hospitals as h
            inner join patient as p on p.hospitalnpi=h.npi
            where p.statecode='21' and h.statecode='21'
            group by h.hospitalname
    ) as t on t.hospitalname=h.hospitalname
    --this where exists clause gives the visitCounts column
    where h.stateCode='21' and p.statecode='21'
    and exists
    (
        select distinct p2.patientid
            from Patient as p2
            inner join Claims as c on c.patientid = p2.patientid
            and c.admissiondate = p2.admissiondate
            and c.dischargedate = p2.dischargedate
            inner join hcpcs on hcpcs.hcpcs=c.hcpcs
            inner join hospitals as h on h.npi=p2.hospitalnpi
            where (hcpcs.hcpcs ='97001' or hcpcs.hcpcs='9339' or hcpcs.hcpcs='97002')
            and p2.patientid=p.patientid 
    ) 
    and hospitalcounts > 10
    group by h.hospitalname, t.hospitalcounts
    having count(*)>10

Answer 1

You might look into CTE (Common Table Expressions) to get what you need. It would allow you to get summarized data and join that back to the detail on a common key. As an example I modified your join on the subquery to be a CTE.

;with hospitalCounts as (
    select h.hospitalname, count(*) as hospitalCounts
    from hospitals as h
    inner join patient as p on p.hospitalnpi=h.npi
    where p.statecode='21' and h.statecode='21'
    group by h.hospitalname
)
select  h.hospitalname
    ,count(*) as visitCounts
    ,hospitalcounts
    ,round(count(*)/cast(hospitalcounts as float) *100,2) as percentRehabNotHomeHealth
from Patient p
inner join statecounties as sc on sc.countycode = p.countycode
and sc.statecode = p.statecode
inner join hospitals as h on h.npi=p.hospitalnpi
inner join hospitalCounts on t.hospitalname=h.hospitalname
--this where exists clause gives the visitCounts column
where h.stateCode='21' and p.statecode='21'
and exists
(
    select p2.patientid
        from Patient as p2
        inner join Claims as c on c.patientid = p2.patientid
        and c.admissiondate = p2.admissiondate
        and c.dischargedate = p2.dischargedate
        inner join hcpcs on hcpcs.hcpcs=c.hcpcs
        inner join hospitals as h on h.npi=p2.hospitalnpi
        where (hcpcs.hcpcs ='97001' or hcpcs.hcpcs='9339' or hcpcs.hcpcs='97002')
        and p2.patientid=p.patientid 
) 
and hospitalcounts > 10
group by h.hospitalname, t.hospitalcounts
having count(*)>10