CREATE TABLE table_204_158 (id DECIMAL, "official name" TEXT, "designation" TEXT, "area km2" DECIMAL, "population" DECIMAL, "parish" TEXT)

which city has the least area
SELECT "official name" FROM table_204_158 ORDER BY "area km2" LIMIT 1
SELECT "official name" FROM "table_204_158" ORDER BY "area km2" NULLS FIRST LIMIT 1