Link subsequent patient visits from same table in SQL - sql

Question

I have a table containing records for patient admissions to a group of hospitals.
I would like to be able to link each record to the most recent previous record for each patient, if there is a previous record or return a null field if there is no previous record.
Further to this I would like to place some criteria of the linked records eg previous visit to the same hospital only, previous visit was less than 7 days before.
The data looks something like this (with a whole lots of other fields)
Record PatientID hospital Admitdate DischargeDate
1. 1. A. 1/2/12. 3/2/12
2. 2. A. 1/2/12. 4/2/12
3. 1. B. 4/3/12. 4/3/12
My thinking was a self join but I can't figure out how to join to the record where the difference between the admit date and the patient's previous discharge date is the minimum.
Thanks!

Answer 1

You could use row_number() to assign increasing numbers to records for each patient. Then you can left join to the previous record:
; with numbered_records as
(
select row_number() over (partition by PatientID, Hospital
order by Record desc) as rn
, *
from YourTable
)
select *
from numbered_records cur
left join
numbered_records prev
on prev.PatientID = cur.PatientID
and prev.Hospital = cur.Hospital
and prev.DischargeDate >= dateadd(day, -7, getdate())
and prev.rn = cur.rn + 1
To select only the latest row per patient, add:
where cur.rn = 1
at the end of the query.

Answer 2

It will give you the First 2 records of the same patients. If you want the same Hospital then add another check of Hospital with the PatientID. Also can add the Date as well.
SELECT * FROM T1 t
WHERE (2 >= (SELECT Count(*) FROM T1 tmp
WHERE t.PatientID = tmp.PatientID
AND t.Record <= tmp.Record))
It will only bring the one record if there is only one entry.

Answer 3

Note that:
I used DATE for data type. It might be possible that a patient visits one hospital before noon, and another in the afternoon. You would use DATETIME in that case. Sorting on the partitioning uses dt_admit before record_id, to allow for entry of data in any order.
CREATE TABLE #hdata(
record_id INT NOT NULL IDENTITY(1,1) PRIMARY KEY,
patient_id INT NOT NULL,
hospital_id INT NOT NULL,
dt_admit DATE NOT NULL,
dt_discharge DATE NULL
);
INSERT INTO #hdata(
patient_id,
hospital_id,
dt_admit,
dt_discharge
)
VALUES (
1,
1,
'2012-02-01',
'2012-02-03'
), (
2,
1,
'2012-02-01',
'2012-02-04'
), (
1,
2,
'2012-03-04',
'2012-03-04'
);
-- 1/ link each record to the previous record for each patient, NULL if none
SELECT
record_id,
patient_id,
ROW_NUMBER() OVER (PARTITION BY patient_id ORDER BY dt_admit,record_id) AS visit_seq_id
INTO
#visit_sequence
FROM
#hdata;
SELECT
v1.record_id,
v1.patient_id,
v2.record_id AS previous_record_id
FROM
#visit_sequence AS v1
LEFT JOIN #visit_sequence AS v2 ON
v2.patient_id=v1.patient_id AND
v2.visit_seq_id=v1.visit_seq_id-1
ORDER BY
v1.record_id;
DROP TABLE #visit_sequence;
-- 2/ criteria on linked records: same hospital, previous visit < 7 days
SELECT
record_id,
patient_id,
hospital_id,
dt_admit,
ROW_NUMBER() OVER (PARTITION BY patient_id,hospital_id ORDER BY dt_admit,record_id) AS visit_seq_id
INTO
#visit_sequence_elab
FROM
#hdata;
SELECT
v1.record_id,
v1.patient_id,
v2.record_id AS previous_record_id
FROM
#visit_sequence_elab AS v1
LEFT JOIN #visit_sequence_elab AS v2 ON
v2.patient_id=v1.patient_id AND
v2.hospital_id=v1.hospital_id AND
v2.visit_seq_id=v1.visit_seq_id-1 AND
DATEDIFF(DAY,v1.dt_admit,v2.dt_admit)<7
ORDER BY
v1.record_id;
DROP TABLE #visit_sequence_elab;
DROP TABLE #hdata;