SQL to determine multiple date ranges (SQL Server 2000)

Question

I have a table which contains an ID and a Date for an event. Each row is for one date. I am trying to determine consecutive date ranges and consoli

Answer 1

This is the way I've done it in the past. It's a two step process:

1. Build the set of candidate contiguous periods
2. If there are any overlapping periods, delete all but the longest such period.

Here's a script that shows how it's done. You might be able to pull it off in a single [bug, ugly] query, but trying to do that makes my head hurt. I'm using temp tables as it makes the debugging a whole lot easier.

drop table #source
create table #source
(
  id    int      not null ,
  dtCol datetime not null ,

  -----------------------------------------------------------------------
  -- ASSUMPTION 1: Each date must be unique for a given ID value.
  -----------------------------------------------------------------------
  unique clustered ( id , dtCol ) ,

  -----------------------------------------------------------------------
  -- ASSUMPTION 2: The datetime column only represents a day.
  -- The value of the time component is always 00:00:00.000
  -----------------------------------------------------------------------
  check ( dtCol = convert(datetime,convert(varchar,dtCol,112),112) ) ,

)
go

insert #source values(1,'jan 1, 2011')
insert #source values(1,'jan 4, 2011')
insert #source values(1,'jan 5, 2011')
insert #source values(2,'jan 1, 2011')
insert #source values(2,'jan 2, 2011')
insert #source values(2,'jan 3, 2011')
insert #source values(2,'jan 5, 2011')
insert #source values(3,'jan 1, 2011')
insert #source values(4,'jan 1, 2011')
insert #source values(4,'jan 2, 2011')
insert #source values(4,'jan 3, 2011')
insert #source values(4,'jan 4, 2011')
go

insert #source values( 200236 , '2011-01-02')
insert #source values( 200236 , '2011-01-03')
insert #source values( 200236 , '2011-01-05')
insert #source values( 200236 , '2011-01-06')
insert #source values( 200236 , '2011-01-07')
insert #source values( 200236 , '2011-01-08')
insert #source values( 200236 , '2011-01-09')
insert #source values( 200236 , '2011-01-10')
insert #source values( 200236 , '2011-01-11')
insert #source values( 200236 , '2011-01-12')
insert #source values( 200236 , '2011-01-13')
insert #source values( 200236 , '2011-01-15')
insert #source values( 200236 , '2011-01-16')
insert #source values( 200236 , '2011-01-17')
go

drop table #candidate_range
go
create table #candidate_range
(
  rowId   int      not null identity(1,1) ,
  id      int      not null ,
  dtFrom  datetime not null ,
  dtThru  datetime not null ,
  length  as 1+datediff(day,dtFrom,dtThru) ,

  primary key nonclustered ( rowID ) ,
  unique clustered (id,dtFrom,dtThru) ,

)
go

--
-- seed the candidate range table with the set of all possible contiguous ranges for each id
--
insert #candidate_range ( id , dtFrom , dtThru )
select id      = tFrom.id    ,
       valFrom = tFrom.dtCol ,
       valThru = tThru.dtCol
from #source tFrom
join #source tThru on tThru.id     = tFrom.id
                  and tThru.dtCol >= tFrom.dtCol
where 1+datediff(day,tFrom.dtCol,tThru.dtCol) = ( select count(*)
                                                  from #source t
                                                  where t.id = tFrom.id
                                                    and t.dtCol between tFrom.dtCol and tThru.dtCol
                                                )
order by 1,2,3
go

--
-- compare the table to itself. If we find overlapping periods,
-- we'll keep the longest such period and delete the shorter overlapping periods.
--
delete t2
from #candidate_range t1
join #candidate_range t2 on t2.id      = t1.id
                        and t2.rowId  != t1.rowID
                        and t2.length <  t1.length
                        and t2.dtFrom <= t1.dtThru
                        and t2.dtThru >= t1.dtFrom
go

That's about all there is to it.