If yongshun is the county what is the overall percentage of the chinas tujia population?
CREATE TABLE "by_county" (
  "province" text,
  "prefecture" text,
  "county" text,
  "tujia_population" real,
  "pct_of_chinas_tujia_population" text
);
SELECT "pct_of_chinas_tujia_population" FROM "by_county" WHERE "county"='Yongshun';