My first real SQL query in PostGIS. It subsets the geo-referenced ethnic groups (GREG) data to only groups that are present in Afghanistan (instead of including hundreds of groups for the entire world).

subset.sql

CREATE TABLE afg_greg AS
SELECT * 
  FROM greg 
  WHERE ( ST_contains((SELECT the_geom FROM afg_adm0), geom) OR
          ST_overlaps((SELECT the_geom FROM afg_adm0), geom) );