Q:

Merge polygons in CartoDB?

I created this map in cartodb: http://cdb.io/1xBj4Xo
Now I have to merge some of the districts to a bigger district. Is it possible to do this in Cartodb? Using a SQL query for instance?
I can also provide a .csv-file: http://tradukt.ch/owncloud/public.php?service=files&t=2fe413428dd38103705fa4191236b8fc
Screenshot of the table:

A:

Based on your CSV, 
Tablename: Wahlkreise
Column containing value to union on: wahlkreis
Other Columns use an aggregate function to grab one of the values (all of them are the same for each district / wahlkreis
In the SQL window in CartoDB, type:
   SELECT   

wahlkreis  
, cartodb_id  
, max(strong_party) as strong_party  
, max(einwohner) as einwohner  
, max(mandate) as mandate  
, max(kandidaten) as kandidaten  
, max(frauen) as frauen  
, max(maenner) as maenner  
, max(fdp) as fdp  
, max(svp) as svp  
, max(sp) as sp  
, max(glp) as glp  
, max(gruene) as gruene  
, max(evp) as evp  
, max(cvp) as cvp  
, max(bdp) as bdp  

, st_union(the_geom_webmercator) as the_geom_webmercator      

FROM wahlkreise_bl_1

group by wahlkreis, wahlkreise_bl_1.cartodb_id

At this point, map interaction must be enabled by adding the CartoDB_id to the SQL and GROUP BY clause, but there seems to be a bug that removes the st_union function from working properly (I will keep digging in).
The workflow instead could be to simply use the 'create table from query' option to write your query to a new file, and you can take advantage of the full interactivity based on your SQL processing.

