CREATE TABLE table_204_678 (id DECIMAL, "record" TEXT, "athlete" TEXT, "nation" TEXT, "venue" TEXT, "date" TEXT, "#" DECIMAL)

which country is the only one to have only one record holder for pole vaulting ?
SELECT "nation" FROM table_204_678 GROUP BY "nation" HAVING COUNT(*) = 1
SELECT "nation" FROM "table_204_678" GROUP BY "nation" HAVING COUNT(*) = 1