How to merge polygons with ST_union

Question

I have a similar problem, but I can't get the syntax correct. This refers to the following question:

Merge polygons in CartoDB?

I'm trying to merge multiple polygons into one (in this case, just the 5 NYC boroughs into a single polygon, but for now I'm trying to merge all of them). This doesn't seem to work, as the result is still separate polygons.

SELECT

counties_ny
, cartodb_id
, max(county) as county
, max(fips) as fips
, max(county_fip) as county_fip

, st_union(the_geom_webmercator) as the_geom_webmercator

FROM counties_ny

group by county, counties_ny.cartodb_id

Answer 1

If you only want one output record, remove the GROUP BY clause:

SELECT
1 AS cartodb,
ST_Union(the_geom_webmercator) AS the_geom_webmercator
FROM counties_ny

The output will be a MultiPolygon, probably, since Staten Island and other things cannot be merged due to the water in between.