Islands And Gaps In Sequential Data

I couldn't find a topic suitable for testing this, so I thought I'd start one.

Here is one way to get the islands without a tally table.declare@test table (symbol char(3), dt smalldatetime)

insert@test
select'abc','01/01/1990' union all
select'abc','01/02/1990' union all
select'abc','01/03/1990' union all
select'abc','01/04/1990' union all
select'abc','01/05/1990' union all
select'def','01/03/1990' union all
select'def','01/04/1990' union all
select'def','01/05/1990' union all
select'def','01/06/1990' union all
select'def','01/07/1990' union all
select'ghi','01/01/1990' union all
select'ghi','01/02/1990' union all
select'ghi','01/06/1990' union all
select'ghi','01/07/1990' union all
select'ghi','01/08/1990'

selectsymbol,
min(dt),
max(dt2)
from(
selectt1.symbol,
t1.dt,
t2.dt as dt2,
(select count(distinct t3.symbol) from @test as t3 where t3.symbol < t1.symbol and t3.dt <= t1.dt) AS r
from@test as t1
inner join@test as t2 on t2.symbol = t1.symbol
wheret2.dt - 1 = t1.dt
) as d
group bysymbol,
r
E 12°55'05.25"
N 56°04'39.16"