Merge overlapping date intervals

Question

Is there a better way of merging overlapping date intervals?
The solution I came up with is so simple that now I wonder if someone else has a better idea of how this cou

Answer 1

I was looking for the same solution and came across this post on Combine overlapping datetime to return single overlapping range record.

There is another thread on Packing Date Intervals.

I tested this with various date ranges, including the ones listed here, and it works correctly every time.

---

SELECT 
       s1.StartDate,
       --t1.EndDate 
       MIN(t1.EndDate) AS EndDate
FROM @T s1 
INNER JOIN @T t1 ON s1.StartDate <= t1.EndDate
  AND NOT EXISTS(SELECT * FROM @T t2 
                 WHERE t1.EndDate >= t2.StartDate AND t1.EndDate < t2.EndDate) 
WHERE NOT EXISTS(SELECT * FROM @T s2 
                 WHERE s1.StartDate > s2.StartDate AND s1.StartDate <= s2.EndDate) 
GROUP BY s1.StartDate 
ORDER BY s1.StartDate 

---

The result is:

StartDate  | EndDate
2010-01-01 | 2010-06-13
2010-06-15 | 2010-06-25
2010-06-26 | 2010-08-16
2010-11-01 | 2010-12-31

Answer 2

Here is a solution with just three simple scans. No CTEs, no recursion, no joins, no table updates in a loop, no "group by" — as a result, this solution should scale the best (I think). I think number of scans can be reduced to two, if min and max dates are known in advance; the logic itself just needs two scans — find gaps, applied twice.

declare @datefrom datetime, @datethru datetime

DECLARE @T TABLE (d1 DATETIME, d2 DATETIME)

INSERT INTO @T (d1, d2)

SELECT '2010-01-01','2010-03-31' 
UNION SELECT '2010-03-01','2010-06-13' 
UNION SELECT '2010-04-01','2010-05-31' 
UNION SELECT '2010-06-15','2010-06-25' 
UNION SELECT '2010-06-26','2010-07-10' 
UNION SELECT '2010-08-01','2010-08-05' 
UNION SELECT '2010-08-01','2010-08-09' 
UNION SELECT '2010-08-02','2010-08-07' 
UNION SELECT '2010-08-08','2010-08-08' 
UNION SELECT '2010-08-09','2010-08-12' 
UNION SELECT '2010-07-04','2010-08-16' 
UNION SELECT '2010-11-01','2010-12-31' 

select @datefrom = min(d1) - 1, @datethru = max(d2) + 1 from @t

SELECT 
StartDate, EndDate
FROM
(
    SELECT 
    MAX(EndDate) OVER (ORDER BY StartDate) + 1 StartDate,
    LEAD(StartDate ) OVER (ORDER BY StartDate) - 1 EndDate
    FROM
    (
        SELECT 
        StartDate, EndDate
        FROM
        (
            SELECT 
            MAX(EndDate) OVER (ORDER BY StartDate) + 1 StartDate,
            LEAD(StartDate) OVER (ORDER BY StartDate) - 1 EndDate 
            FROM 
            (
                SELECT d1 StartDate, d2 EndDate from @T 
                UNION ALL 
                SELECT @datefrom StartDate, @datefrom EndDate 
                UNION ALL 
                SELECT @datethru StartDate, @datethru EndDate
            ) T
        ) T
        WHERE StartDate <= EndDate
        UNION ALL 
        SELECT @datefrom StartDate, @datefrom EndDate 
        UNION ALL 
        SELECT @datethru StartDate, @datethru EndDate
    ) T
) T
WHERE StartDate <= EndDate

The result is:

StartDate   EndDate
2010-01-01  2010-06-13
2010-06-15  2010-08-16
2010-11-01  2010-12-31

Answer 3

The idea is to simulate the scanning algorithm for merging intervals. My solution makes sure it works across a wide range of SQL implementations. I've tested it on MySQL, Postgres, SQL-Server 2017, SQLite and even Hive.

Assuming the table schema is the following.

CREATE TABLE t (
  a DATETIME,
  b DATETIME
);

We also assume the interval is half-open like [a,b).

When (a,i,j) is in the table, it shows that there are j intervals covering a, and there are i intervals covering the previous point.

CREATE VIEW r AS 
SELECT a,
       Sum(d) OVER (ORDER BY a ROWS BETWEEN UNBOUNDED PRECEDING AND 1 PRECEDING) AS i,
       Sum(d) OVER (ORDER BY a ROWS UNBOUNDED PRECEDING) AS j
FROM  (SELECT a, Sum(d) AS d
       FROM   (SELECT a,  1 AS d FROM t
               UNION ALL
               SELECT b, -1 AS d FROM t) e
       GROUP  BY a) f;

We produce all the endpoints in the union of the intervals and pair up adjacent ones. Finally, we produce the set of intervals by only picking the odd-numbered rows.

SELECT a, b
FROM (SELECT a,
             Lead(a)      OVER (ORDER BY a) AS b,
             Row_number() OVER (ORDER BY a) AS n
      FROM   r
      WHERE  j=0 OR i=0 OR i is null) e
WHERE  n%2 = 1;

I've created a sample DB-fiddle and SQL-fiddle. I also wrote a blog post on union intervals in SQL.

Answer 4

Try this

;WITH T1 AS
(
    SELECT d1, d2, ROW_NUMBER() OVER(ORDER BY (SELECT 0)) AS R
    FROM @T
), NUMS AS
(
    SELECT ROW_NUMBER() OVER(ORDER BY (SELECT 0)) AS R
    FROM T1 A
    CROSS JOIN T1 B
    CROSS JOIN T1 C
), ONERANGE AS 
(
    SELECT DISTINCT DATEADD(DAY, ROW_NUMBER() OVER(PARTITION BY T1.R ORDER BY (SELECT 0)) - 1, T1.D1) AS ELEMENT
    FROM T1
    CROSS JOIN NUMS
    WHERE NUMS.R <= DATEDIFF(DAY, d1, d2) + 1
), SEQUENCE AS
(
    SELECT ELEMENT, DATEDIFF(DAY, '19000101', ELEMENT) - ROW_NUMBER() OVER(ORDER BY ELEMENT) AS rownum
    FROM ONERANGE
)
SELECT MIN(ELEMENT) AS StartDate, MAX(ELEMENT) as EndDate
FROM SEQUENCE
GROUP BY rownum

The basic idea is to first unroll the existing data, so you get a separate row for each day. This is done in ONERANGE

Then, identify the relationship between how dates increment and the way the row numbers do. The difference remains constant within an existing range/island. As soon as you get to a new data island, the difference between them increases because the date increments by more than 1, while the row number increments by 1.

Answer 5

You asked this back in 2010 but don't specify any particular version.

An answer for people on SQL Server 2012+

WITH T1
     AS (SELECT *,
                MAX(d2) OVER (ORDER BY d1) AS max_d2_so_far
         FROM   @T),
     T2
     AS (SELECT *,
                CASE
                  WHEN d1 <= DATEADD(DAY, 1, LAG(max_d2_so_far) OVER (ORDER BY d1))
                    THEN 0
                  ELSE 1
                END AS range_start
         FROM   T1),
     T3
     AS (SELECT *,
                SUM(range_start) OVER (ORDER BY d1) AS range_group
         FROM   T2)
SELECT range_group,
       MIN(d1) AS d1,
       MAX(d2) AS d2
FROM   T3
GROUP  BY range_group 

Which returns

+-------------+------------+------------+
| range_group |     d1     |     d2     |
+-------------+------------+------------+
|           1 | 2010-01-01 | 2010-06-13 |
|           2 | 2010-06-15 | 2010-08-16 |
|           3 | 2010-11-01 | 2010-12-31 |
+-------------+------------+------------+

DATEADD(DAY, 1 is used because your desired results show you want a period ending on 2010-06-25 to be collapsed into one starting 2010-06-26. For other use cases this may need adjusting.

Answer 6

In this solution, I created a temporary Calendar table which stores a value for every day across a range. This type of table can be made static. In addition, I'm only storing 400 some odd dates starting with 2009-12-31. Obviously, if your dates span a larger range, you would need more values.

In addition, this solution will only work with SQL Server 2005+ in that I'm using a CTE.

With Calendar As
    (
    Select DateAdd(d, ROW_NUMBER() OVER ( ORDER BY s1.object_id ), '1900-01-01') As [Date]
    From sys.columns as s1
        Cross Join sys.columns as s2
    )
    , StopDates As
    (
    Select C.[Date]
    From Calendar As C
        Left Join @T As T
            On C.[Date] Between T.d1 And T.d2
    Where C.[Date] >= ( Select Min(T2.d1) From @T As T2 )
        And C.[Date] <= ( Select Max(T2.d2) From @T As T2 )
        And T.d1 Is Null
    )
    , StopDatesInUse As
    (
    Select D1.[Date]
    From StopDates As D1
        Left Join StopDates As D2
            On D1.[Date] = DateAdd(d,1,D2.Date)
    Where D2.[Date] Is Null
    )
    , DataWithEariestStopDate As 
    (
    Select *
    , (Select Min(SD2.[Date])
        From StopDatesInUse As SD2
        Where T.d2 < SD2.[Date] ) As StopDate
    From @T As T
    )
Select Min(d1), Max(d2)
From DataWithEariestStopDate
Group By StopDate
Order By Min(d1)

EDIT The problem with using dates in 2009 has nothing to do with the final query. The problem is that the Calendar table is not big enough. I started the Calendar table at 2009-12-31. I have revised it start at 1900-01-01.