Table: northumberland_county
Columns: official_name
status
area_km_2
population
census_ranking

Question: What is the average population vlue for an area smaller than 26.69 square km and has an official name of Rogersville?
SQL: SELECT AVG("population") FROM northumberland_county WHERE "area_km_2" < 26.69 AND "official_name" = 'rogersville'