What is the sum of Tujia population with the Zhangjiajie prefecture in Sangzhi county?
CREATE TABLE "by_county" (
  "province" text,
  "prefecture" text,
  "county" text,
  "tujia_population" real,
  "pct_of_china_s_tujia_population" text
);
SELECT SUM("tujia_population") FROM "by_county" WHERE "prefecture"='zhangjiajie' AND "county"='sangzhi';