The UnionAggregate() function groups the polygons and removes the internal boundaries. Because it aggregates the polygon, a GROUP BY clause must be used.

Note the #tt1. This sets up a temporary table on the SQL Server. This makes it easy if combining it with the Polyline Encoded Converter since your Polyline Encoded script can then reference #tt1 in the from statement, similar to a Qlik RESIDENT table.

In the example below, a custom 'Cluster' boundary is being created.

select cluster,
  geometry::UnionAggregate(geom) as geom
into #tt1
from qm_shapes.dbo."Clusters_FY17" c
  inner join qm_shapes.dbo.unsimplified_zip z
  on (c.Zip = z.zip)
group by cluster;