Обсуждение: problematic query (for me ;-)
I'm trying to get a hint as to how i can solve this problem: i have a table: n | t ---+--- 1 | 2 1 | 5 2 | 3 2 | 5 3 | 4 3 | 3 (6 rows) now i want to find the pairs (n1,n2) where no t's collide - eg in this table it would be (1,3) and (3,1). Note - this is not homework ;-) what i'm actually trying to do is to extract n=(countyid,roadid) t=housenumber - and find out which roads that *cannot* cross county border because their housenumbers collide (to this i've already narrowed the n's depending on roadname).
"Kim Petersen" <kp@kyborg.dk> wrote in message news:3E660A41.7040509@kyborg.dk... > I'm trying to get a hint as to how i can solve this problem: > > i have a table: > > n | t > ---+--- > 1 | 2 > 1 | 5 > 2 | 3 > 2 | 5 > 3 | 4 > 3 | 3 > (6 rows) > > now i want to find the pairs (n1,n2) where no t's collide - eg in this > table it would be (1,3) and (3,1). SELECT DISTINCT nt1.n, nt2.n FROM nt nt1, nt nt2 WHERE NOT EXISTS ( (SELECT t from nt where n=nt1.n) INTERSECT (SELECT t from nt where n=nt2.n)) n | n ---+---1 | 33 |1 (2 rows) I'm not sure that it scales well to large datasets... /Bjarke
Kim Petersen wrote: > I'm trying to get a hint as to how i can solve this problem: it seems it works to post to a newsgroup - i've been trying to solve this for hours - and just came up with a solution - only question now is whether or not it is usable (on millons of tuples). select distinct t1.n,t2.n from test t1,test t2 where not exists (select t from test where n=t1.n intersect select t from test where n=t2.n); optimization ideas would be greatly appreciated.