question: What is the highest number of not usable satellites when there are more than 0 launch failures, less than 30 retired, and the block is I?
answer: SELECT MAX(not_usable) FROM table_name_30 WHERE launch_failures > 0 AND retired < 30 AND block = "block i"
context: CREATE TABLE table_name_30 (not_usable INTEGER, block VARCHAR, launch_failures VARCHAR, retired VARCHAR)