[postgis-users] I'm missing something

[Bob Pawley]

Hi

I am attemptijng to use the ST_Union function to collapse three rows from a 
table called tank_lin into one row in table called process_dgm.

Here's what I used -
 Insert into graphics.process_dgm(the_geom)
 Select   ST_Union(the_geom)
 from tank_lin
 Group by tank_lin.the_geom;

The result is three rows inserted into process_dgm, each row appearing to be 
simple copies of each of the original rows.
I also attempted group by another column but received error 'unknown 
geometry'.

I also tried STR_LineMerge with similar results.

What am I not considering?

Bob