Question

I have two tables both of which have columns StartDate and EndDate.

I’m trying to return a single resultset that contains all date ranges from one table (TableA), and all complement date ranges from the other one (TableB).

CREATE TABLE [dbo].[TableA](
    [ID] [int] NOT NULL,
    [StartDate] [datetime] NOT NULL,
    [EndDate] [datetime] NOT NULL
)

CREATE TABLE [dbo].[TableB](
    [ID] [int] NOT NULL,
    [StartDate] [datetime] NOT NULL,
    [EndDate] [datetime] NOT NULL
)

INSERT INTO TableA (ID, StartDate, EndDate) VALUES(1, '4/1/2009', '8/1/2009')
INSERT INTO TableA (ID, StartDate, EndDate) VALUES(1, '10/1/2009', '12/1/2009')
INSERT INTO TableB (ID, StartDate, EndDate) VALUES(1, '1/1/2009', '2/1/2010')

INSERT INTO TableA (ID, StartDate, EndDate) VALUES(2, '4/1/2009', '8/1/2009')
INSERT INTO TableB (ID, StartDate, EndDate) VALUES(2, '1/1/2009', '5/1/2009')
INSERT INTO TableB (ID, StartDate, EndDate) VALUES(2, '7/1/2009', '12/1/2009')

The expected resultset from the three datasets should be:

(ID = 1)
1/1/2009 - 4/1/2009 (from TableB)
4/1/2009 - 8/1/2009 (from TableA)
8/1/2009 - 10/1/2009 (from TableB)
10/1/2009 - 12/1/2009 (from TableA)
12/1/2009 - 2/1/2010 (from TableB)

(ID = 2)
1/1/2009 - 4/1/2009 (from TableB)
4/1/2009 - 8/1/2009 (from TableA)
8/1/2009 - 12/1/2009 (from TableB)

The date ranges are not guaranteed to be continuous, and I can’t make any assumptions on how they’re overlapping between tables…within each table they can be assumed to not overlap.

I’m having problems wrapping my head around how to split the single date ranges in TableB into multiple pieces to find all the complement “regions” within it in SQL.

Anyone have any suggestions?

Answer 1

If you create this as a view, I think it does what you want. It uses CTEs, which should be supported by SQL Server 2005, but not earlier.

WITH Timestamps AS (
    SELECT Id, StartDate AS Date FROM TableA
    UNION
    SELECT Id, EndDate AS Date FROM TableA
    UNION
    SELECT Id, StartDate AS Date FROM TableB
    UNION
    SELECT Id, EndDate AS Date FROM TableB
), Timestamps2 AS (
    SELECT ROW_NUMBER() OVER (ORDER BY Id, Date) AS RowNumber, * FROM Timestamps
), Timestamps3 AS (
    SELECT T1.ID, T1.Date AS StartDate, T2.Date AS EndDate
    FROM Timestamps2 AS T1 JOIN Timestamps2 AS T2
    ON T1.RowNumber + 1 = T2.RowNumber AND T1.ID = T2.ID
), IntervalsFromB AS (
    SELECT T.ID, T.StartDate, T.EndDate FROM Timestamps3 AS T
    LEFT JOIN TableA AS A
    ON T.StartDate >= A.StartDate AND T.EndDate <= A.EndDate
    WHERE A.StartDate IS NULL)
SELECT * FROM TableA
UNION ALL
SELECT * FROM IntervalsFromB

Full output (ordered by Id, StartDate for readability):

Id  StartDate               EndDate
1   2009-01-01 00:00:00.000 2009-04-01 00:00:00.000
1   2009-04-01 00:00:00.000 2009-08-01 00:00:00.000
1   2009-08-01 00:00:00.000 2009-10-01 00:00:00.000
1   2009-10-01 00:00:00.000 2009-12-01 00:00:00.000
1   2009-12-01 00:00:00.000 2010-02-01 00:00:00.000
2   2009-01-01 00:00:00.000 2009-04-01 00:00:00.000
2   2009-04-01 00:00:00.000 2009-08-01 00:00:00.000
2   2009-08-01 00:00:00.000 2009-12-01 00:00:00.000

It was pretty complicated for me to implement this, so I’m wondering if anyone can see a simpler way. I might be missing some trick that makes this much simpler. If so, please let me know! Also, you will almost certainly need some indexes on your tables to get this to perform well if you have a lot of rows. Some other optimizations may be possible – I haven’t tried for the fastest possible performance, but just to get the correct result.