I want to merge some lines with conditions but nothing seems to done . Here is the query :

drop table if exists touches_diameter_material;
create table touches_diameter_material as select a.*,st_union(a.geom,b.geom) 
as fusion from pipe a, pipe b where a.id > b.id and  a.material = b.material 
and a.diameter = b.diameter   and st_touches(a.geom,b.geom)

if you know how to do it I'll be pleased to get some help.

  • what conditions exactly? and "nothing seems to done"? what are you referring to? – ziggy Sep 28 '17 at 15:25
  • Nomraly when I execute st_union the number of lines of the resulting table shoud be less than the one before union yet I still have the same number of records you see what I mean. Like the union wasn't executed or he didn't take in consideration the where clauses I put – zakaria mouqcit Sep 28 '17 at 15:32
  • 1
    im surprised the query doesnt throw an error that you are not using a group by with the st_union function – ziggy Sep 28 '17 at 15:41

With ST_Union, you need a GROUP BY statement, otherwise everything will be combined and you will receive a single record only.

    st_union(a.geom,b.geom) as fusion
FROM pipe a, pipe b
WHERE a.id > b.id
AND  a.material = b.material 
AND a.diameter = b.diameter
AND st_touches(a.geom,b.geom)
GROUP by a.id, a.material, a.diameter
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