Condense Time Periods with SQL

Question

I have a large data set which for the purpose of this question has 3 fields:

• Group Identifier
• From Date
• To Date

On any given row t

Answer 1

I'd use a Calendar table. This table simply has a list of dates for several decades.

CREATE TABLE [dbo].[Calendar](
    [dt] [date] NOT NULL,
CONSTRAINT [PK_Calendar] PRIMARY KEY CLUSTERED 
(
    [dt] ASC
))

There are many ways to populate such table.

For example, 100K rows (~270 years) from 1900-01-01:

INSERT INTO dbo.Calendar (dt)
SELECT TOP (100000) 
    DATEADD(day, ROW_NUMBER() OVER (ORDER BY s1.[object_id])-1, '19000101') AS dt
FROM sys.all_objects AS s1 CROSS JOIN sys.all_objects AS s2
OPTION (MAXDOP 1);

Once you have a Calendar table, here is how to use it.

Each original row is joined with the Calendar table to return as many rows as there are dates between From and To.

Then possible duplicates are removed.

Then classic gaps-and-islands by numbering the rows in two sequences.

Then grouping found islands together to get the new From and To.

Sample data

I added a second group.

DECLARE @T TABLE (GroupID int, FromDate date, ToDate date);
INSERT INTO @T (GroupID, FromDate, ToDate) VALUES
(1, '2012-01-01', '2012-12-31'),
(1, '2013-12-01', '2014-11-30'),
(1, '2015-01-01', '2015-12-31'),
(1, '2015-01-01', '2015-12-31'),
(1, '2015-02-01', '2015-03-31'),
(1, '2013-01-01', '2013-12-31'),
(2, '2012-01-01', '2012-12-31'),
(2, '2013-01-01', '2013-12-31');

Query

WITH
CTE_AllDates
AS
(
    SELECT DISTINCT
        T.GroupID
        ,CA.dt
    FROM
        @T AS T
        CROSS APPLY
        (
            SELECT dbo.Calendar.dt
            FROM dbo.Calendar
            WHERE
                dbo.Calendar.dt >= T.FromDate
                AND dbo.Calendar.dt <= T.ToDate
        ) AS CA
)
,CTE_Sequences
AS
(
    SELECT
        GroupID
        ,dt
        ,ROW_NUMBER() OVER(PARTITION BY GroupID ORDER BY dt) AS Seq1
        ,DATEDIFF(day, '2001-01-01', dt) AS Seq2
        ,DATEDIFF(day, '2001-01-01', dt) - 
            ROW_NUMBER() OVER(PARTITION BY GroupID ORDER BY dt) AS IslandNumber
    FROM CTE_AllDates
)
SELECT
    GroupID
    ,MIN(dt) AS NewFromDate
    ,MAX(dt) AS NewToDate
FROM CTE_Sequences
GROUP BY GroupID, IslandNumber
ORDER BY GroupID, NewFromDate;

Result

+---------+-------------+------------+
| GroupID | NewFromDate | NewToDate  |
+---------+-------------+------------+
|       1 | 2012-01-01  | 2014-11-30 |
|       1 | 2015-01-01  | 2015-12-31 |
|       2 | 2012-01-01  | 2013-12-31 |
+---------+-------------+------------+