What is the sum of Silver when the total is less than 6, the rank is 6 and the Bulgaria is the nation?
CREATE TABLE "medal_table" (
  "rank" text,
  "nation" text,
  "gold" real,
  "silver" real,
  "bronze" real,
  "total" real
);
SELECT SUM("silver") FROM "medal_table" WHERE "total"<6 AND "rank"='6' AND "nation"='bulgaria';