What is Population density (per km²), when Name is Antigua and Barbuda, and when Capital is St. John's?
CREATE TABLE "sovereign_states" (
  "name" text,
  "subdivisions" text,
  "area_km" real,
  "population_1_july_2005_est" real,
  "population_density_per_km" real,
  "capital" text
);
SELECT "population_density_per_km" FROM "sovereign_states" WHERE "name"='antigua and barbuda' AND "capital"='st. john''s';