Identify ranges in groups which are identified by a flag - sql-server

Question

I have the following table:
declare #table table (dates int , is_missing tinyint, group_id numeric(18))
insert into #table(dates,is_missing,group_id)
select 20110719,0,1
union all
select 20110720,0,1
union all
select 20110721,0,1
union all
select 20110722,1,1
union all
select 20110723,0,1
union all
select 20110724,0,1
union all
select 20110725,0,1
union all
select 20110726,1,1
union all
select 20110727,0,1
union all
select 20110728,1,1
union all
select 20110723,1,3
union all
select 20110724,0,3
union all
select 20110725,0,3
union all
select 20110726,1,3
union all
select 20110727,0,3
select * from #table
order by group_id, dates
What I am trying to do is to return ranges of dates for each group which are identified by the missing day flag. To make this more clear the results of the query will have to look like this:
group_id start_date end_date days_count
1 20110719 20110721 3
1 20110723 20110725 3
1 20110727 20110727 1
3 20110724 20110725 2
3 20110727 20110727 1
The is_missing flag basicaly separates the ranges per group. It actually says that a date is missing and therefore all the other dates located between is_missing flags are the groups I am trying to find their start and end dates as well as their days numbers count.
Is there a simple way to do this?
Thanks a lot.

Answer 1

Here is a possible solution using Common Table Expression (CTE) and ROW_NUMBER(). This type of problem is known as islands. Using the concept that was used in this Stack Overflow question: sql group by only rows which are in sequence, the following query was formulated to produce desired output against the data provided by you.
This query works correctly if the data stored in the table is ordered by group_id and dates columns. I assume that is the case with your data. If not, you might need to tweak the solution.
Modified the query as per suggestions provided by Andriy M. Thanks to Andriy M.
The query has been changed so that it can provide correct output even if the date values in the table are not in sequence. The question has the date values stored in int data type instead of date format. So, two queries have been provided below. First query will work if the table contains date values stored in int data typeand the second query will work if the table contains date values stored in datetime or date data type.
This query will work only in SQL Server versions 2005 and above. Since you have tagged your question under sql-server-2008, I think this should work for you.
Screenshot #1 displays the data stored in the table. Screenshot #2 displays the output of the below mentioned queries against the table data.
Hope that helps.
Query for date values stored in int data type:
.
WITH cte AS
(
SELECT datenumeric
, is_missing
, group_id
, datenumeric
- DENSE_RANK() OVER (PARTITION BY is_missing ORDER BY group_id, datenumeric) AS partition_grp
FROM dbo.table_data
)
SELECT cte.group_id
, MIN(cte.datenumeric) AS start_date
, MAX(cte.datenumeric) AS end_date
, COUNT(cte.datenumeric) AS days_count
FROM cte
WHERE cte.is_missing = 0
GROUP BY cte.group_id
, cte.partition_grp
ORDER BY cte.group_id
, cte.partition_grp;
Query for date values stored in datetime or date data type:
.
WITH cte AS
(
SELECT datevalue
, is_missing
, group_id
, DATEDIFF(DAY, 0, datevalue)
- DENSE_RANK() OVER (PARTITION BY is_missing ORDER BY group_id, datevalue) AS partition_grp
FROM dbo.table_data
)
SELECT cte.group_id
, MIN(cte.datevalue) AS start_date
, MAX(cte.datevalue) AS end_date
, COUNT(cte.datevalue) AS days_count
FROM cte
WHERE cte.is_missing = 0
GROUP BY cte.group_id
, cte.partition_grp
ORDER BY cte.group_id
, cte.partition_grp;
Screenshot #1:
Screenshot #2:

Answer 2

With many thanks to Siva for the nice solution, I thought if there was one date missing in the data, the query would fail.
so I modified the query a little and used ROW_NUMBER() to fix that.
WITH cte AS
(
SELECT dates
, is_missing
, group_id
,ROW_NUMBER() OVER (ORDER BY group_id, dates) -
DENSE_RANK() OVER (PARTITION BY is_missing ORDER BY group_id, dates) AS partition_Id
FROM dbo.table_data
)
SELECT group_id
, MIN(dates) AS start_date
, MAX(dates) AS end_date
, COUNT(*) AS days_count
FROM cte
WHERE is_missing = 0
GROUP BY group_id
, partition_id
ORDER BY group_id
, partition_id;
Or maybe a missing date will never happen. :)