Créer une géotable :

CREATE TABLE test (id serial PRIMARY KEY, genre text);
SELECT AddGeometryColumn( 'test', 'the_geom', -1, 'GEOMETRY', 2 );

Alimenter la table avec quelques géométries en dur et pas vraiment “spatiales” (SRS = -1 !!):

INSERT INTO test VALUES ( 3, 'batiment1', ST_GeometryFromText( 'POLYGON((10 10,40 20,35 8,12 4,10 10))', -1 ) ); 
INSERT INTO test VALUES ( 4, 'batiment2', ST_GeometryFromText( 'POLYGON((10 40,20 30,30 40,40 35,50 60,35 80,20 60,10 40))', -1 ) ); 
INSERT INTO test VALUES ( 5, 'batiment3', ST_GeometryFromText( 'POLYGON((10 95,20 95,20 135,10 135,10 95))', -1 ) ); 

INSERT INTO test VALUES ( 1, 'pieton1', ST_GeometryFromText( 'POINT(10 70)', -1 ) ); 
INSERT INTO test VALUES ( 2, 'pieton2', ST_GeometryFromText( 'POINT(30 30)', -1 ) ); 
INSERT INTO test VALUES ( 6, 'pieton3', ST_GeometryFromText( 'POINT(35 70)', -1 ) ); 
INSERT INTO test VALUES ( 7, 'pieton4', ST_GeometryFromText( 'POINT(35 60)', -1 ) ); 

INSERT INTO test VALUES ( 8, 'bordureRoute1', ST_GeometryFromText( 'LINESTRING(1 85,50 85)', -1 ) ); 
INSERT INTO test VALUES ( 9, 'bordureRoute2', ST_GeometryFromText( 'LINESTRING(1 92,50 92)', -1 ) ); 

(→ cf. documentation PostGIS Reference)

1. Quelles sont les aires des objets ? → ST_Area2d

   cf. 2

2. Quel sont les types géométriques des objets ? → Geometrytype

   SELECT *, ST_Area(the_geom) FROM test WHERE ST_Geometrytype(the_geom) = 'ST_Polygon'

3. Quel est l'objet géométrique le plus proche du pieton 2 ? → ST_Distance

   SELECT genre, ST_Distance(the_geom, (SELECT the_geom FROM test WHERE id = 2)) as dist FROM test 
   WHERE id <> 2 ORDER BY dist LIMIT 1
   
   OU
   
   SELECT t1.* FROM test t1, test t2 WHERE t2.genre LIKE 'pieton2'
    AND ST_Distance(t1.the_geom,t2.the_geom) = (
     SELECT min(ST_Distance(t1.the_geom, t2.the_geom)) FROM test t1, test t2 
     WHERE t1.genre LIKE 'pieton2' AND t2.genre <> 'pieton2'
    )

4. Qui est dans le bâtiment 2 ? → ST_Within

   SELECT * FROM test WHERE genre like 'pieton%'
   AND ST_WithIn(test.the_geom, (SELECT the_geom FROM test WHERE genre LIKE 'batiment2'));

(source: Jean David Techer)