where value world rank is 12, what is the quantity world rank?
CREATE TABLE "table1_21109892_1" (
  "rank" real,
  "commodity" text,
  "value_int_1000" real,
  "production_mt" real,
  "quantity_world_rank" text,
  "value_world_rank" text
);
SELECT "quantity_world_rank" FROM "table1_21109892_1" WHERE "value_world_rank"='12';