postgresql semi-join

country_code_europe.sql

select *
from country
where country.code in
    (select code
     from country
     where population > 7*1000*1000)
  and country.continent = 'Europe';

                             QUERY PLAN                               
------------------------------------------------------------------------
 Hash Join  (cost=8.56..17.08 rows=18 width=113)
   Hash Cond: (country_1.code = country.code)
   ->  Seq Scan on country country_1  (cost=0.00..7.99 rows=93 width=4)
         Filter: (population > 7000000)
   ->  Hash  (cost=7.99..7.99 rows=46 width=113)
         ->  Seq Scan on country  (cost=0.00..7.99 rows=46 width=113)
               Filter: (continent = 'Europe'::text)
(7 rows)