Question

I have my table (cte) defintions and result set here

The CTE may look strange but it has been tested and returns the correct results in the most efficient manner that I’ve found yet. The below query will find the number of person IDs (patid) who are taking two or more drugs at the same time. Currently, the query works insofar as it returns the patIDs of the people taking both drugs, but not both drugs at the same time. Taking both drugs is indicated by one fillDate of one drug falling before a scriptEndDate of another drug. So

You can see in this partial result set that on line 18 the scriptFillDate is 2009-07-19 which is between the fillDate and scriptEndDate of the same patID from row 2. What constraint do I need to add so I can filter these unneeded results?

--PatientDrugList is a CTE because eventually parameters might be passed to it
--to alter the selection population
;with PatientDrugList(patid, filldate, scriptEndDate,drugName,strength)
as
(
    select rx.patid,rx.fillDate,rx.scriptEndDate,rx.drugName,rx.strength
        from rx
),
--the row constructor here will eventually be parameters for a stored procedure
DrugList (drugName)
as
(
    select x.drugName
        from (values ('concerta'),('fentanyl'))
        as x(drugName)
        where x.drugName is not null
)


    --the row number here is so that I can find the largest date range
    --(the largest datediff means the person was on a given drug for a larger
    --amount of time.  obviously not a optimal solution
     --celko inspired relational division!
     select distinct row_number() over(partition by pd.patid, drugname order by datediff(day,pd.fillDate,pd.scriptEndDate)desc)  as rn
     ,pd.patid
    ,pd.drugname
    ,pd.fillDate
    ,pd.scriptEndDate
    from PatientDrugList as pd
    where not exists
    (select * from DrugList 
    where not exists
    (select * from PatientDrugList as pd2
    where(pd.patid=pd2.patid)
    and (pd2.drugName = DrugList.drugName)))
    and exists 
    (select * 
        from DrugList
        where DrugList.drugName=pd.drugName
    )
    group by pd.patid, pd.drugName,pd.filldate,pd.scriptEndDate

Answer 1

Wrap you original query into a CTE, or better yet, for performance, stability of query plan and result, store it into a temp table.

The query below (assuming CTE option) will give you the overlapping times when both drugs are being taken.

;with tmp as (
   .. your query producing the columns shown ..
)
select *
  from tmp a
  join tmp b on a.patid = b.patid and a.drugname <> b.drugname
 where a.filldate < b.scriptenddate
   and b.filldate < a.scriptenddate;