SELECT (st_union(geom)) FROM lands WHERE 1=1;
SELECT ST_AsGeoJSON(st_transform(geom , 4326)) as geojson FROM farms WHERE 2=2 LIMIT 1;
SELECT st_asgeojson(st_envelope(ST_Union(st_transform(geom,4326)))) from xxx WHERE 1=1
原文：http://www.cnblogs.com/restful/p/7009021.html