SQL: Gaps and Islands, Grouped dates

Question

I am trying to group dates within 3 days of each other and assign points based on readmission\'s within 30 days. A MRN would receive 3 points per readmission. Any help on mo

Answer 1

This answer works for the example you provided. It might be useful if you have small tables and limited admission. (It use recursion on the dates).

WITH a AS (
    SELECT
        z1.VisitDate
        , z1.OrganizationMrn
        , (SELECT MIN(VisitDate) FROM #z WHERE VisitDate > DATEADD(day, 3, z1.VisitDate)) AS NextDay
    FROM
        #z z1
    WHERE
        CATEGORY = 'Inpatient'
), a1 AS ( 
    SELECT
        OrganizationMrn
        , MIN(VisitDate) AS VisitDate
        , MIN(NextDay) AS NextDay
    FROM
        a
    GROUP BY
        OrganizationMrn
), b AS (
    SELECT
        VisitDate
        , OrganizationMrn
        , NextDay
        , 1 AS OrderRow
    FROM
        a1


    UNION ALL

    SELECT
        a.VisitDate
        , a.OrganizationMrn
        , a.NextDay
        , b.OrderRow +1 AS OrderRow
    FROM
        a
        JOIN b
        ON a.VisitDate = b.NextDay
), c AS (
SELECT
    VisitDate
    , (SELECT MAX(VisitDate) FROM b WHERE b1.VisitDate > VisitDate) AS PreviousVisitDate
FROM
    b b1
)
SELECT
    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