What is the mean huc example code when the example name's lower snake, there are 6 digits, and less than 3 levels?
CREATE TABLE "united_states" (
  "name" text,
  "level" real,
  "digits" real,
  "average_size_square_miles" text,
  "number_of_h_us_approximate" real,
  "example_name" text,
  "example_code_huc" real
);
SELECT AVG("example_code_huc") FROM "united_states" WHERE "example_name"='lower snake' AND "digits"=6 AND "level"<3;