CREATE TABLE table_203_515 (id DECIMAL, "rank" DECIMAL, "city" TEXT, "passengers" DECIMAL, "ranking" DECIMAL, "airline" TEXT)

which airline carries the most passengers ?
SELECT "airline" FROM table_203_515 ORDER BY "passengers" DESC LIMIT 1
SELECT "airline" FROM "table_203_515" ORDER BY "passengers" DESC NULLS LAST LIMIT 1