SQL: CTE query Speed - sql

Question

I am using SQL Server 2008 and am trying to increase the speed of my query below. The query assigns points to patients based on readmission dates.
Example: A patient is seen on 1/2, 1/5, 1/7, 1/8, 1/9, 2/4. I want to first group visits within 3 days of each other. 1/2-5 are grouped, 1/7-9 are grouped. 1/5 is NOT grouped with 1/7 because 1/5's actual visit date is 1/2. 1/7 would receive 3 points because it is a readmit from 1/2. 2/4 would also receive 3 points because it is a readmit from 1/7. When the dates are grouped the first date is the actual visit date.
Most articles suggest limiting the data set or adding indexes to increase speed. I have limited the amount of rows to about 15,000 and added a index. When running the query with 45 test visit dates/ 3 test patients, the query takes 1.5 min to run. With my actual data set it takes > 8 hrs.
How can I get this query to run < 1 hr? Is there a better way to write my query? Does my Index look correct? Any help would be greatly appreciated.
Example expected results below query.
;CREATE TABLE RiskReadmits(MRN INT, VisitDate DATE, Category VARCHAR(15))
;CREATE CLUSTERED INDEX Risk_Readmits_Index ON RiskReadmits(VisitDate)
;INSERT RiskReadmits(MRN,VisitDate,CATEGORY)
VALUES
(1, '1/2/2016','Inpatient'),
(1, '1/5/2016','Inpatient'),
(1, '1/7/2016','Inpatient'),
(1, '1/8/2016','Inpatient'),
(1, '1/9/2016','Inpatient'),
(1, '2/4/2016','Inpatient'),
(1, '6/2/2016','Inpatient'),
(1, '6/3/2016','Inpatient'),
(1, '6/5/2016','Inpatient'),
(1, '6/6/2016','Inpatient'),
(1, '6/8/2016','Inpatient'),
(1, '7/1/2016','Inpatient'),
(1, '8/1/2016','Inpatient'),
(1, '8/4/2016','Inpatient'),
(1, '8/15/2016','Inpatient'),
(1, '8/18/2016','Inpatient'),
(1, '8/28/2016','Inpatient'),
(1, '10/12/2016','Inpatient'),
(1, '10/15/2016','Inpatient'),
(1, '11/17/2016','Inpatient'),
(1, '12/20/2016','Inpatient')
;WITH a AS (
SELECT
z1.VisitDate
, z1.MRN
, (SELECT MIN(VisitDate) FROM RiskReadmits WHERE VisitDate > DATEADD(day, 3, z1.VisitDate)) AS NextDay
FROM
RiskReadmits z1
WHERE
CATEGORY = 'Inpatient'
), a1 AS (
SELECT
MRN
, MIN(VisitDate) AS VisitDate
, MIN(NextDay) AS NextDay
FROM
a
GROUP BY
MRN
), b AS (
SELECT
VisitDate
, MRN
, NextDay
, 1 AS OrderRow
FROM
a1
UNION ALL
SELECT
a.VisitDate
, a.MRN
, a.NextDay
, b.OrderRow +1 AS OrderRow
FROM
a
JOIN b
ON a.VisitDate = b.NextDay
), c AS (
SELECT
MRN,
VisitDate
, (SELECT MAX(VisitDate) FROM b WHERE b1.VisitDate > VisitDate AND b.MRN = b1.MRN) AS PreviousVisitDate
FROM
b b1
)
SELECT distinct
c1.MRN,
c1.VisitDate
, CASE
WHEN DATEDIFF(day,c1.PreviousVisitDate,c1.VisitDate) < 30 THEN PreviousVisitDate
ELSE NULL
END AS ReAdmissionFrom
, CASE
WHEN DATEDIFF(day,c1.PreviousVisitDate,c1.VisitDate) < 30 THEN 3
ELSE 0
END AS Points
FROM
c c1
ORDER BY c1.MRN
Expected Results:
MRN VisitDate ReAdmissionFrom Points
1 2016-01-02 NULL 0
1 2016-01-07 2016-01-02 3
1 2016-02-04 2016-01-07 3
1 2016-06-02 NULL 0
1 2016-06-06 2016-06-02 3
1 2016-07-01 2016-06-06 3
1 2016-08-01 NULL 0
1 2016-08-15 2016-08-01 3
1 2016-08-28 2016-08-15 3
1 2016-10-12 NULL 0
1 2016-11-17 NULL 0
1 2016-12-20 NULL 0

Answer 1

oops I changed the names of a few cte's (and the post messed up what was code)
It should be like this:
b AS (
SELECT
VisitDate
, MRN
, NextDay
, 1 AS OrderRow
FROM
a1
UNION ALL
SELECT
a.VisitDate
, a.MRN
, a.NextDay
, b.OrderRow +1 AS OrderRow
FROM
a AS a
JOIN b
ON a.VisitDate = b.NextDay AND a.MRN = b.MRN
)

Answer 2

I'm going to take a wild guess here and say you want to change the b cte to
have AND a.MRN = b.MRN as a second condition in the second select query like this:
, b AS (
SELECT
VisitDate
, MRN
, NextDay
, 1 AS OrderRow
FROM
firstVisitAndFollowUp
UNION ALL
SELECT
a.VisitDate
, a.MRN
, a.NextDay
, b.OrderRow +1 AS OrderRow
FROM
visitsDistance3daysOrMore AS a
JOIN b
ON a.VisitDate = b.NextDay AND a.MRN = b.MRN
)