What was the 2006 population for the local government area of alexandrina council whose rank was smaller than 19?
CREATE TABLE "25_largest_local_government_areas_by_pop" (
  "rank" real,
  "local_government_area" text,
  "population_2006_census" real,
  "population_2007_estimate" real,
  "growth_rate" text,
  "stat_division_district" text
);
SELECT AVG("population_2006_census") FROM "25_largest_local_government_areas_by_pop" WHERE "local_government_area"='alexandrina council' AND "rank"<19;