CREATE TABLE competition_record (competition_id DECIMAL, farm_id DECIMAL, rank DECIMAL)
CREATE TABLE farm (farm_id DECIMAL, year DECIMAL, total_horses DECIMAL, working_horses DECIMAL, total_cattle DECIMAL, oxen DECIMAL, bulls DECIMAL, cows DECIMAL, pigs DECIMAL, sheep_and_goats DECIMAL)
CREATE TABLE farm_competition (competition_id DECIMAL, year DECIMAL, theme TEXT, host_city_id DECIMAL, hosts TEXT)
CREATE TABLE city (city_id DECIMAL, official_name TEXT, status TEXT, area_km_2 DECIMAL, population DECIMAL, census_ranking TEXT)

What are the themes of farm competitions sorted by year in ascending order?
SELECT theme FROM farm_competition ORDER BY year
SELECT "theme" FROM "farm_competition" ORDER BY "year" NULLS FIRST