[postgis-users] basic functionalities of PostGIS

[ΑΠΟΣΤΟΛΟΣ ΛΕΛΕΝΤΖΗΣ]

I try to execute the basic functionalities of GIS (union, sym_difference,
buffer, intersect...) with PostGIS 1.5, but i encounter some problems.
I create a new table to store results of overlay analysis, and i observe
that every entity of new thematic layer is triplicate!!
Specifically, i want to union two layers which include 15 and 3 entities
respectively, and i use the following command:
   CREATE TABLE tablename  AS
   SELECT ST_UNION (a.the_geom, b.the_geom)
   FROM table1 AS a, table2 AS b ;
and i note that the above SQL query relate each entity from one layer with
each entity from other, like a cross join!!!
What happened??
Also, i find that the new thematic layer isn't add to the table
"geometry_columns"!!!Why??

Thanks,
Apostolis
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