CREATE TABLE table_204_311 (id DECIMAL, "rank" DECIMAL, "nation" TEXT, "gold" DECIMAL, "silver" DECIMAL, "bronze" DECIMAL, "total" DECIMAL)

which nation won the most silver medals mexico or spain ?
SELECT "nation" FROM table_204_311 WHERE "nation" IN ('mexico', 'spain') ORDER BY "silver" DESC LIMIT 1
SELECT "nation" FROM "table_204_311" WHERE "nation" IN ('mexico', 'spain') ORDER BY "silver" DESC NULLS LAST LIMIT 1