I had thought of inner join but could not figure out what condition would follow ON (there seemed to be nothing!). I had forgotten that ROWID tables have a column that I could use. I have been looking at the window function ROW_NUMBER() (not sure if this is more reliable than rowid). Your first solution shortened (less to do means quicker to finish, usually) my code. Your present solution has the ON clause. My 'real' table consists of the administrative boundaries of 265 countries, with the polygon for Canada having (the most points) 255,955 points or vertices. I need to do some more tests (& timings); this might take a while not least because I am looking for means of simplifying-read reducing the number of vertices- the polygons. SpatiaLite does not have a Reduce method for simplifying polygons i.e. reducing the number of vertices - any one know the equivalent method with SpatiaLite? Thanks for sharing your insights, everyone.