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Home -> Community -> Usenet -> comp.databases.theory -> Re: SQL competition: the shortest interval coalesce/pack query
"Mikito Harakiri" <mikharakiri_at_iahu.com> wrote in message news:<2uPrd.61$G45.36_at_news.oracle.com>...
> Write the shortest sql query that would return a minimal cover of a set of
> intervals. For example, given
>
> Intervals = {(x=1,y=3), (x=2,y=5), (x=4, y=11), (x=10,y=12), (x=20,y=21)}
>
> it should return
>
> cover(Intervals) = {(x=1,y=12), (x=20,y=21)}
>
> "Developing Time-Oriented..." book by R.Snordgrass demonstrates several
> rather lenthy solutions (pp.160+). The book is freely downloadable.
>
> To be able to compare results, lets agree that each of the clauses (select,
> from, where, group by, having) starts a new line. (Each subquery/inner view
> must obey this rule as well.) Also, no more than 2 column expressions are
> allowed to fit on a single line in the select clause. Likewise, no more than
> 2 view/tables are allowed on a single line within from clause, no more than
> 2 predicates in the each line of the where clause, etc. If anybody knows
> alternative (maybe even standard) sql formatting rules, you are welcome to
> suggest those.
I have not looked at the article, so I'm not sure this is any shorter but:
with cover(x,y,n) as (
select x, y, 1 from intervals
union all
select c.x, i.y, n+1 from cover c, intervals i
where c.y > i.x
and (n+1) < (select count(1) from intervals)
)
select x, max(y) from cover c
where not exists (
select 1 from cover where y=c.y and x < c.x ) group by x
tested with
create table intervals (
x int not null, y int not null, primary key (x,y)
insert into intervals (x,y) values (1,3),(2,5),(4,11),(10,12),(20,21);
/Lennart Received on Fri Dec 03 2004 - 14:28:02 CST