[postgis-users] Documentation on GIS specific SQL

[David Blasby]

Matt,

The reason you're having problems is because extent() is an aggregate
function - like min() or max().

To convert a geometry to a bounding box:

SELECT geom::box3d FROM <Table>
or
SELECT BOX3D(geom) FROM <table>

Use extent() when you want to find the COMBINED bounding box of several
geometries:

SELECT extent(property_polygon) FROM property WHERE owner = 'dblasby';
or
SELECT owner, extent(property_polygon)  FROM property GROUP BY owner;

This of this like the min() and max() functions:

SELECT max(salary) FROM employees WHERE title = 'analyst';
SELECT title,max(salary) FROM employees GROUP BY title;