A customer have a table with data stored by dates, and asked us to present data from this table by sequential date ranges.

This query sounded trivial but took us half a day to create such a select.

For simplicity consider a table of integer numbers, and try to build a select that returns pairs of continuous ranges of values.

So, for an input like this:

declare @values table
(
  value int not null primary key
);

insert into @values(value)
select  1 union all select  2 union all select  3 union all
select  5 union all select  6 union all
select  8 union all
select 10 union all
select 12 union all select 13 union all select 14;

You will have a following output:

low  high
---- ----
1    3
5    6
8    8
10   10
12   14

Logic of the algorithms is like this:

1. get a low bound of each range (a value without value - 1 in the source);
2. get a high bound of each range (a value without value + 1 in the source);
3. combine low and high bounds.

Following this logic we have built at least three different queries, where the shortest one is:

with source as
(
  select * from @values
)
select
  l.value low,
  min(h.value) high
from
  source l
  inner join
  source h
  on
    (l.value - 1 not in (select value from source)) and
    (h.value + 1 not in (select value from source)) and
    (h.value >= l.value)
group by
  l.value;

Looking at this query it's hard to understand why it took so long to write so simple code...