Table: south_australia_places
Columns: rank
local_government_area
population_2006_census
population_2007_estimate
growth_rate
stat_division_district

Question: What was the 2006 population for the local government area of alexandrina council whose rank was smaller than 19?
SQL: SELECT AVG("population_2006_census") FROM south_australia_places WHERE "local_government_area" = 'alexandrina council' AND "rank" < 19