SQL Server episode identification - sql

Question

I am working with a blood pressure database in SQL Server which contains patient_id, timestamp (per minute) and systolicBloodPressure.
My goals are to find:
the number of episodes in which a patient is under a certain blood pressure threshold
An episode consists of the timestmap where the patient drops below a certain threshold until the timestamp where the patient comes above the threshold.
the mean blood pressure per episode per patient
the duration of the episode per episode per patient
What I have tried so far:
I am able to identify episodes by just making a new column which sets to 1 if threshold is reached.
select *
, CASE
when sys < threshold THEN '1'
from BPDATA
However , I am not able to 'identify' different episodes within the patient; episode1 episode 2 with their relative timestamps.
Could someone help me with this? Or is there someone with a better different solution?
EDIT: Sample data with example threshold 100
ID Timestamp SysBP below Threshold
----------------------------------------------------
1 9:38 110 Null
1 9:39 105 Null
1 9:40 96 1
1 9:41 92 1
1 9:42 102 Null
2 12:23 95 1
2 12:24 98 1
2 12:25 102 Null
2 12:26 104 Null
2 12:27 94 1
2 12:28 88 1
2 12:29 104 Null

Answer 1

Thanks for the sample data.
This should work:
declare #t table (ID int, Timestamp time, SysBP int, belowThreshold bit)
insert #t
values
(1, '9:38', 110, null),
(1, '9:39', 105, null),
(1, '9:40', 96, 1),
(1, '9:41', 92, 1),
(1, '9:42', 102, null),
(2, '12:23', 95, 1),
(2, '12:24', 98, 1),
(2, '12:25', 102, null),
(2, '12:26', 104, null),
(2, '12:27', 94, 1),
(2, '12:28', 88, 1),
(2, '12:29', 104, null)
declare #treshold int = 100
;with y as (
select *, case when lag(belowThreshold, 1, 0) over(partition by id order by timestamp) = belowThreshold then 0 else 1 end epg
from #t
),
z as (
select *, sum(epg) over(partition by id order by timestamp) episode
from y
where sysbp < #treshold
)
select id, episode, count(episode) over(partition by id) number_of_episodes_per_id, avg(sysbp) avg_sysbp, datediff(minute, min(timestamp), max(timestamp))+1 episode_duration
from z
group by id, episode

Answer 2

This answer relies on LEAD() and LAG() functions so only works on 2012 or later:
Setup:
CREATE TABLE #bloodpressure
(
Patient_id int,
[TimeStamp] SmallDateTime,
SystolicBloodPressure INT
)
INSERT INTO #bloodpressure
VALUES
(1, '2017-01-01 09:01', 60),
(1, '2017-01-01 09:02', 55),
(1, '2017-01-01 09:03', 60),
(1, '2017-01-01 09:04', 70),
(1, '2017-01-01 09:05', 72),
(1, '2017-01-01 09:06', 75),
(1, '2017-01-01 09:07', 60),
(1, '2017-01-01 09:08', 50),
(1, '2017-01-01 09:09', 52),
(1, '2017-01-01 09:10', 53),
(1, '2017-01-01 09:11', 65),
(1, '2017-01-01 09:12', 71),
(1, '2017-01-01 09:13', 73),
(1, '2017-01-01 09:14', 74),
(2, '2017-01-01 09:01', 70),
(2, '2017-01-01 09:02', 75),
(2, '2017-01-01 09:03', 80),
(2, '2017-01-01 09:04', 70),
(2, '2017-01-01 09:05', 72),
(2, '2017-01-01 09:06', 75),
(2, '2017-01-01 09:07', 60),
(2, '2017-01-01 09:08', 50),
(2, '2017-01-01 09:09', 52),
(2, '2017-01-01 09:10', 53),
(2, '2017-01-01 09:11', 65),
(2, '2017-01-01 09:12', 71),
(2, '2017-01-01 09:13', 73),
(2, '2017-01-01 09:14', 74),
(3, '2017-01-01 09:12', 71),
(3, '2017-01-01 09:13', 60),
(3, '2017-01-01 09:14', 74)
Now using Lead And Lag to find the previous rows values, to find whether this is the beginning or end of a sequence of low blood pressures, in combination with a common table expression. Using a UNION of start and end events ensures that an event which covers just one minute is recorded as both a start and an end event.
;WITH CTE
AS
(
SELECT *,
LAG(SystolicBloodPressure,1)
OVER (PaRTITION BY Patient_Id ORDER BY TimeStamp) As PrevValue,
Lead(SystolicBloodPressure,1)
OVER (PaRTITION BY Patient_Id ORDER BY TimeStamp) As NextValue
FROM #bloodpressure
),
CTE2
AS
(
-- Get Start Events (EventType 1)
SELECT 1 As [EventType], Patient_id, TimeStamp,
ROW_NUMBER() OVER (ORDER BY Patient_id, TimeStamp) AS RN
FROM CTE
WHERE (PrevValue IS NULL AND SystolicBloodPressure < 70) OR
(PrevValue >= 70 AND SystolicBloodPressure < 70)
UNION
-- Get End Events (EventType 2)
SELECT 2 As [EventType], Patient_id, TimeStamp,
ROW_NUMBER() OVER (ORDER BY Patient_id, TimeStamp) AS RN
FROM CTE
WHERE (NextValue IS NULL AND SystolicBloodPressure < 70 ) OR
(NextValue >= 70 AND SystolicBloodPressure < 70)
)
SELECT C1.Patient_id, C1.TimeStamp As EventStart, C2.TimeStamp As EventEnd
FROM CTE2 C1
INNER JOIN CTE2 C2
ON C1.Patient_id = C2.Patient_id AND C1.RN = C2.RN
WHERE C1.EventType = 1 AND C2.EventType = 2
ORDER BY C1.Patient_id, C1.TimeStamp