How to merge time intervals in SQL Server

Question

Suppose I have the following an event table with personId, startDate and endDate.

I want to know how much time the person X sp

Answer 1

Edit 1: I have modified both solutions to get correct results.

Edit 2: I have done comparative tests using the solutions proposed by Mikael Eriksson, Conrad Frix, Philip Kelley and me. All tests use an EventTable with the following structure:

CREATE TABLE EventTable
(
     EventID    INT IDENTITY PRIMARY KEY
    ,PersonId   INT NOT NULL
    ,StartDate  DATETIME NOT NULL
    ,EndDate    DATETIME NOT NULL
    ,CONSTRAINT CK_StartDate_Before_EndDate CHECK(StartDate < EndDate)
);

Also, all tests use warm buffer (no DBCC DROPCLEANBUFFERS) and cold [plan] cache (I have executed DBCC FREEPROCCACHE before every test). Because some solutions use a filter(PersonId = 1) and others not, I have inserted into EventTable rows for only one person (INSERT ...(PersonId,...) VALUES (1,...)).

These are the results:

My solutions use recursive CTEs.

Solution 1:

WITH BaseCTE
AS
(
    SELECT   e.StartDate
            ,e.EndDate
            ,e.PersonId
            ,ROW_NUMBER() OVER(PARTITION BY e.PersonId ORDER BY e.StartDate, e.EndDate) RowNumber
    FROM    EventTable e
),  RecursiveCTE
AS
(
    SELECT   b.PersonId
            ,b.RowNumber

            ,b.StartDate
            ,b.EndDate
            ,b.EndDate AS MaxEndDate
            ,1 AS PseudoDenseRank
    FROM    BaseCTE b
    WHERE   b.RowNumber = 1
    UNION ALL
    SELECT   crt.PersonId
            ,crt.RowNumber

            ,crt.StartDate
            ,crt.EndDate
            ,CASE WHEN crt.EndDate > prev.MaxEndDate THEN crt.EndDate ELSE prev.MaxEndDate END
            ,CASE WHEN crt.StartDate <= prev.MaxEndDate THEN prev.PseudoDenseRank ELSE prev.PseudoDenseRank + 1 END
    FROM    RecursiveCTE prev
    INNER JOIN BaseCTE crt ON prev.PersonId = crt.PersonId
    AND     prev.RowNumber + 1 = crt.RowNumber
),  SumDaysPerPersonAndInterval
AS
(
    SELECT   src.PersonId
            ,src.PseudoDenseRank --Interval ID
            ,DATEDIFF(DAY, MIN(src.StartDate), MAX(src.EndDate)) Days
    FROM    RecursiveCTE src
    GROUP BY src.PersonId, src.PseudoDenseRank
)
SELECT  x.PersonId, SUM( x.Days ) DaysPerPerson
FROM    SumDaysPerPersonAndInterval x
GROUP BY x.PersonId
OPTION(MAXRECURSION 32767);

Solution 2:

DECLARE @Base TABLE --or a temporary table: CREATE TABLE #Base (...) 
(
     PersonID   INT NOT NULL
    ,StartDate  DATETIME NOT NULL
    ,EndDate    DATETIME NOT NULL
    ,RowNumber  INT NOT NULL
    ,PRIMARY KEY(PersonID, RowNumber)
);
INSERT  @Base (PersonID, StartDate, EndDate, RowNumber)
SELECT   e.PersonId
        ,e.StartDate
        ,e.EndDate
        ,ROW_NUMBER() OVER(PARTITION BY e.PersonID ORDER BY e.StartDate, e.EndDate) RowNumber
FROM    EventTable e;

WITH RecursiveCTE
AS
(
    SELECT   b.PersonId
            ,b.RowNumber

            ,b.StartDate
            ,b.EndDate
            ,b.EndDate AS MaxEndDate
            ,1 AS PseudoDenseRank
    FROM    @Base b
    WHERE   b.RowNumber = 1
    UNION ALL
    SELECT   crt.PersonId
            ,crt.RowNumber

            ,crt.StartDate
            ,crt.EndDate
            ,CASE WHEN crt.EndDate > prev.MaxEndDate THEN crt.EndDate ELSE prev.MaxEndDate END
            ,CASE WHEN crt.StartDate <= prev.MaxEndDate THEN prev.PseudoDenseRank ELSE prev.PseudoDenseRank + 1 END
    FROM    RecursiveCTE prev
    INNER JOIN @Base crt ON prev.PersonId = crt.PersonId
    AND     prev.RowNumber + 1 = crt.RowNumber
),  SumDaysPerPersonAndInterval
AS
(
    SELECT   src.PersonId
            ,src.PseudoDenseRank --Interval ID
            ,DATEDIFF(DAY, MIN(src.StartDate), MAX(src.EndDate)) Days
    FROM    RecursiveCTE src
    GROUP BY src.PersonId, src.PseudoDenseRank
)
SELECT  x.PersonId, SUM( x.Days ) DaysPerPerson
FROM    SumDaysPerPersonAndInterval x
GROUP BY x.PersonId
OPTION(MAXRECURSION 32767);