MSSQL: Create incremental row label per group - sql-server

Question

In my table, I have a primary key and a date. What I'd like to achieve is to have an incremental label based on whether or not there is a break between the dates - column Goal.
Now, below is an example. The break column was calculated using LEAD function (I thought it might help).
I am able to solve it using T-SQL, but this would be last resort. Nothing I tried has worked so far. I am using MSSQL 2014.
PK | Date | break | Goal |
-------------------------------
1 | 03/2017 | 0 | 1 |
1 | 04/2017 | 0 | 1 |
1 | 08/2017 | 1 | 2 |
1 | 09/2017 | 0 | 2 |
1 | 10/2017 | 0 | 2 |
1 | 02/2018 | 1 | 3 |
1 | 03/2018 | 0 | 3 |
Here is a code to reproduce this example:
CREATE TABLE #test
(
ConsumerId INT,
FullDate DATE,
Goal INT
)
INSERT INTO #test (ConsumerId, FullDate, Goal) VALUES (1,'2017-03-01',1)
INSERT INTO #test (ConsumerId, FullDate, Goal) VALUES (1,'2017-04-01',1)
INSERT INTO #test (ConsumerId, FullDate, Goal) VALUES (1,'2017-08-01',2)
INSERT INTO #test (ConsumerId, FullDate, Goal) VALUES (1,'2017-09-01',2)
INSERT INTO #test (ConsumerId, FullDate, Goal) VALUES (1,'2017-10-01',2)
INSERT INTO #test (ConsumerId, FullDate, Goal) VALUES (1,'2018-02-01',3)
INSERT INTO #test (ConsumerId, FullDate, Goal) VALUES (1,'2018-03-01',3)
SELECT ConsumerId,
FullDate,
CASE WHEN (datediff(month,
isnull(
LEAD (FullDate,1) OVER (PARTITION BY ConsumerId ORDER BY FullDate DESC),
FullDate),
FullDate) > 1)
THEN 1
ELSE 0
END AS break,
Goal
FROM #test
ORDER BY FullDate ASC
EDIT
This is apparently a famous problem "Islands and gaps" as pointed out in the comments. And Google offers many solutions as well as other questions here at SO.

Answer 1

Try this...
WITH
cte_TestGap AS (
SELECT
t.ConsumerId, t.FullDate,
Gap = CASE
WHEN DATEDIFF(mm, t.FullDate, LAG(t.FullDate, 1) OVER (PARTITION BY t.ConsumerId ORDER BY t.FullDate)) = -1
THEN 0
ELSE ROW_NUMBER() OVER (PARTITION BY t.ConsumerId ORDER BY t.FullDate)
END
FROM
#test t
),
cte_SmearGap AS (
SELECT
tg.ConsumerId, tg.FullDate,
GV = MAX(tg.Gap) OVER (PARTITION BY tg.ConsumerId ORDER BY tg.FullDate ROWS UNBOUNDED PRECEDING)
FROM
cte_TestGap tg
)
SELECT
sg.ConsumerId, sg.FullDate,
GroupValue = DENSE_RANK() OVER (PARTITION BY sg.ConsumerId ORDER BY sg.GV)
FROM
cte_SmearGap sg;
An explanation of the code an how it works...
The 1st query, in cte_TestGap, uses the LAG function along with ROW_NUMBER() function to mark the location of gap in the data. We can see that by breaking it out and looking at it's results...
WITH
cte_TestGap AS (
SELECT
t.ConsumerId, t.FullDate,
Gap = CASE
WHEN DATEDIFF(mm, t.FullDate, LAG(t.FullDate, 1) OVER (PARTITION BY t.ConsumerId ORDER BY t.FullDate)) = -1
THEN 0
ELSE ROW_NUMBER() OVER (PARTITION BY t.ConsumerId ORDER BY t.FullDate)
END
FROM
#test t
)
SELECT * FROM cte_TestGap;
cte_TestGap results...
ConsumerId FullDate Gap
----------- ---------- --------------------
1 2017-03-01 1
1 2017-04-01 0
1 2017-08-01 3
1 2017-09-01 0
1 2017-10-01 0
1 2018-02-01 6
1 2018-03-01 0
At this point we want the 0 values to take on the value of the preceding non-0 values, allowing them to be grouped together. This is done in the 2nd query (cte_SmearGap) using the MAX function with a "window frame". So if we look at the output of cte_SmearGap, we can see that...
WITH
cte_TestGap AS (
SELECT
t.ConsumerId, t.FullDate,
Gap = CASE
WHEN DATEDIFF(mm, t.FullDate, LAG(t.FullDate, 1) OVER (PARTITION BY t.ConsumerId ORDER BY t.FullDate)) = -1
THEN 0
ELSE ROW_NUMBER() OVER (PARTITION BY t.ConsumerId ORDER BY t.FullDate)
END
FROM
#test t
),
cte_SmearGap AS (
SELECT
tg.ConsumerId, tg.FullDate,
GV = MAX(tg.Gap) OVER (PARTITION BY tg.ConsumerId ORDER BY tg.FullDate ROWS UNBOUNDED PRECEDING)
FROM
cte_TestGap tg
)
SELECT * FROM cte_SmearGap;
cte_SmearGap results...
ConsumerId FullDate GV
----------- ---------- --------------------
1 2017-03-01 1
1 2017-04-01 1
1 2017-08-01 3
1 2017-09-01 3
1 2017-10-01 3
1 2018-02-01 6
1 2018-03-01 6
At this point All of the rows are in distinct groups... but... We'd like to have our group numbers in a contiguous sequence (1,2,3) as opposed to (1,3,6).
Of course that's easy enough to fix using the DENSE_Rank() function, which is what's happening in the final select...
WITH
cte_TestGap AS (
SELECT
t.ConsumerId, t.FullDate,
Gap = CASE
WHEN DATEDIFF(mm, t.FullDate, LAG(t.FullDate, 1) OVER (PARTITION BY t.ConsumerId ORDER BY t.FullDate)) = -1
THEN 0
ELSE ROW_NUMBER() OVER (PARTITION BY t.ConsumerId ORDER BY t.FullDate)
END
FROM
#test t
),
cte_SmearGap AS (
SELECT
tg.ConsumerId, tg.FullDate,
GV = MAX(tg.Gap) OVER (PARTITION BY tg.ConsumerId ORDER BY tg.FullDate ROWS UNBOUNDED PRECEDING)
FROM
cte_TestGap tg
)
SELECT
sg.ConsumerId, sg.FullDate,
GroupValue = DENSE_RANK() OVER (PARTITION BY sg.ConsumerId ORDER BY sg.GV)
FROM
cte_SmearGap sg;
The end result...
ConsumerId FullDate GroupValue
----------- ---------- --------------------
1 2017-03-01 1
1 2017-04-01 1
1 2017-08-01 2
1 2017-09-01 2
1 2017-10-01 2
1 2018-02-01 3
1 2018-03-01 3

Answer 2

The comment from David Browne was actually extremely useful. If you google "Islands and Gaps", there are many variations of the solution. Below is the one I liked the most.
In the end, I needed the Goal column to be able to group the dates into MIN/MAX. This solution skips this step and directly creates the aggregated range.
Here is the source.
SELECT MIN(FullDate) AS range_start,
MAX(FUllDate) AS range_end
FROM (
SELECT FullDate,
DATEADD(MM, -1 * ROW_NUMBER() OVER(ORDER BY FullDate), FullDate) AS grp
FROM #test
) a
GROUP BY a.grp
And the output:
range_start | range_end |
--------------------------
2017-03-01 | 2017-04-01 |
2017-08-01 | 2017-10-01 |
2018-02-01 | 2018-03-01 |