[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
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