Find overlapping prescriptions - sql-server-2008

Question

I have a bunch of patient prescriptions, each having a certain start date and an end date. I would like to find the instances where a patient has been taking more than one drug in the same drug category for more than 2 days. duration should overlap.
Table structure looks like this:
PatientID StartDate EndDate Drug DrugCategory
1 1/1/2013 1/5/2013 A Cat1
1 1/1/2013 1/4/2013 B Cat1
1 1/10/2013 1/12/2013 C Cat1
2 ....... ........ ............. .........
As seen above, Patient-1 was prescribed 3 drugs in the same category and the first two drug overlapped in duration more than 2 days. So, for this example, I would like the query return the first two records for Patient-1 along with drug name, patientid.
Hope someone can help. This is using SQL Server 2008 R2 btw.

Answer 1

Do you want them as separate rows or as one row? If you want them as separate rows, this should work; otherwise you can pivot the result.
create table want as
select H.* from have H, have V
where H.drug ne V.drug
and H.PatientID=V.PatientID
and H.startDate <= V.startDate
and V.startDate <= H.endDate-2
union select V.* from have H, have V
where H.drug ne V.drug
and H.PatientID=V.PatientID
and H.startDate <= V.startDate
and V.startDate <= H.endDate-2
;
I union the H and V records, I'm sure there's a more efficient way to do that but couldn't easily come up with one. (Just H works for the example given, but for a more proper example where the start dates are not always equal you need the V row as well.)

Answer 2

Please test this thoroughly before using. I have tested it and so far it looks good but not done an exhaustive testing. Would be great if you can test it out further sufficient to your requirements and can point any anomalies in the result if present or take it forward and modify if required.
--Test data:
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
SET ANSI_PADDING ON
GO
CREATE TABLE [dbo].[Prescriptions](
[PatientID] [int] NULL,
[StartDate] [datetime] NULL,
[EndDate] [datetime] NULL,
[Drug] [varchar](50) NULL,
[DrugCategory] [varchar](50) NULL
) ON [PRIMARY]
GO
SET ANSI_PADDING OFF
GO
INSERT [dbo].[Prescriptions] ([PatientID], [StartDate], [EndDate], [Drug], [DrugCategory]) VALUES (1, CAST(0x0000A13A00000000 AS DateTime), CAST(0x0000A13E00000000 AS DateTime), N'D', N'Cat1')
INSERT [dbo].[Prescriptions] ([PatientID], [StartDate], [EndDate], [Drug], [DrugCategory]) VALUES (1, CAST(0x0000A13A00000000 AS DateTime), CAST(0x0000A13B00000000 AS DateTime), N'E', N'Cat1')
INSERT [dbo].[Prescriptions] ([PatientID], [StartDate], [EndDate], [Drug], [DrugCategory]) VALUES (1, CAST(0x0000A13800000000 AS DateTime), CAST(0x0000A13B00000000 AS DateTime), N'F', N'Cat1')
INSERT [dbo].[Prescriptions] ([PatientID], [StartDate], [EndDate], [Drug], [DrugCategory]) VALUES (1, CAST(0x0000A13800000000 AS DateTime), CAST(0x0000A13900000000 AS DateTime), N'G', N'Cat1')
INSERT [dbo].[Prescriptions] ([PatientID], [StartDate], [EndDate], [Drug], [DrugCategory]) VALUES (1, CAST(0x0000A12300000000 AS DateTime), CAST(0x0000A13900000000 AS DateTime), N'Z', N'Cat1')
INSERT [dbo].[Prescriptions] ([PatientID], [StartDate], [EndDate], [Drug], [DrugCategory]) VALUES (1, CAST(0x0000A12300000000 AS DateTime), CAST(0x0000A13A00000000 AS DateTime), N'Y', N'Cat1')
INSERT [dbo].[Prescriptions] ([PatientID], [StartDate], [EndDate], [Drug], [DrugCategory]) VALUES (1, CAST(0x0000A13900000000 AS DateTime), CAST(0x0000A13D00000000 AS DateTime), N'A', N'Cat1')
INSERT [dbo].[Prescriptions] ([PatientID], [StartDate], [EndDate], [Drug], [DrugCategory]) VALUES (1, CAST(0x0000A13900000000 AS DateTime), CAST(0x0000A13C00000000 AS DateTime), N'B', N'Cat1')
INSERT [dbo].[Prescriptions] ([PatientID], [StartDate], [EndDate], [Drug], [DrugCategory]) VALUES (1, CAST(0x0000A14200000000 AS DateTime), CAST(0x0000A14400000000 AS DateTime), N'C', N'Cat1')
Query Used:
SELECT DISTINCT PatientID,StartDate,EndDate,Drug,DrugCategory FROM (
SELECT
DATEDIFF(dd,a.startdate,b.startdate) c1
,DATEDIFF(dd,a.enddate,b.enddate)c2
,DATEDIFF(dd,a.startdate,b.enddate) c3
,DATEDIFF(dd,a.enddate,b.startdate) c4
,DATEDIFF(dd,a.startdate,b.enddate)+DATEDIFF(dd,a.enddate,b.startdate) c34
,a.PatientID
,a.StartDate
,a.EndDate
,a.Drug
,a.DrugCategory
,b.PatientID AS PatientID1
,b.StartDate AS StartDate1
,b.EndDate AS EndDate1
,b.Drug AS Drug1
,b.DrugCategory DrugCategory1
FROM Prescriptions a
,Prescriptions b
WHERE a.patientid=b.patientid
AND a.DrugCategory= b.DrugCategory
and a.drug<>b.drug
)a
WHERE c1*c2*c3*c4 <0
AND c3>2
and c4<=-2
ORDER BY 1,2,3,4
Results:
PatientID StartDate EndDate Drug DrugCategory
----------- ----------------------- ----------------------- -------------------------------------------------- --------------------------------------------------
1 2012-12-10 00:00:00.000 2013-01-02 00:00:00.000 Y Cat1
1 2012-12-31 00:00:00.000 2013-01-03 00:00:00.000 F Cat1
1 2013-01-01 00:00:00.000 2013-01-04 00:00:00.000 B Cat1
1 2013-01-01 00:00:00.000 2013-01-05 00:00:00.000 A Cat1
1 2013-01-02 00:00:00.000 2013-01-06 00:00:00.000 D Cat1
(5 row(s) affected)

Answer 3

what you should be able to do is join the prescription table to itself on the patientID and drugCategory fields where the drugName differs and the startDate OR endDate of the 2nd instance spans the startDate/endDate of the first. Then, determine the overlap range by subtracting the number of days between the max(startDates) and min(endDates). If the overlap is greater than 2 days, return the row:
select *, datediff(d, start_max, end_min) as overlap
from (
SELECT
P.PatientID, P.StartDate, P.EndDate, P.Drug, P.DrugCategory,
P1.StartDate AS p1_start, P1.EndDate AS p1_end, P1.Drug AS p1_drug,
CASE WHEN p.startdate >= P1.startdate THEN p.startdate ELSE P1.startdate END AS start_max,
CASE WHEN p.EndDate <= P1.EndDate THEN p.EndDate ELSE P1.EndDate END AS end_min
FROM
dbo.Prescriptions p INNER JOIN
dbo.Prescriptions AS P1 ON
P.PatientID = P1.PatientID AND
P.DrugCategory = P1.DrugCategory AND
P.Drug <> P1.Drug
WHERE
(P1.StartDate >= P.StartDate AND P1.StartDate <= P.EndDate) OR
(P1.EndDate >= P.StartDate AND P1.EndDate <= P.EndDate)
) t
where
datediff(d, start_max, end_min) > 2