[postgis-users] Aggregate Functions pramsey at refractions.net
Thu Feb 20 09:48:30 PST 2003 What is an aggregate function? A
function which works on a result set and returns a summary. For
example, select max(salary) from employees select mean(age) from
patients Both queries return a *single* value, the aggregate
result. What is a spatial aggregate? Same thing, only with
goemetries. I really look forward to being able to do this to
create a single drainage polygon for a river system: select
union(geom) from watersheds where ws_code > '1000331421' Things
might get confusing, because the opengis spatial operators will
have the same name but different arguments. select
union(a.geom,b.geom) from a,b where a.id = b.id The standard
union() function is a pairwise union. I hope that confuses
things substantially. :/ | Paul Ramsey | Refractions Research |
Email: pramsey at refractions.net | Phone: (250) 885-0632 More
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