question: What is the mean huc example code when the example name's lower snake, there are 6 digits, and less than 3 levels?
answer: SELECT AVG(example_code__huc_) FROM table_name_35 WHERE example_name = "lower snake" AND digits = 6 AND level < 3
context: CREATE TABLE table_name_35 (example_code__huc_ INTEGER, level VARCHAR, example_name VARCHAR, digits VARCHAR)