CREATE TABLE chip_model (model_name TEXT, launch_year DECIMAL, ram_mib DECIMAL, rom_mib DECIMAL, slots TEXT, wifi TEXT, bluetooth TEXT)
CREATE TABLE screen_mode (graphics_mode DECIMAL, char_cells TEXT, pixels TEXT, hardware_colours DECIMAL, used_kb DECIMAL, map TEXT, type TEXT)
CREATE TABLE phone (company_name TEXT, hardware_model_name TEXT, accreditation_type TEXT, accreditation_level TEXT, date TEXT, chip_model TEXT, screen_mode TEXT)

Find the average ram mib size of the chip models that are never used by any phone.
SELECT AVG(ram_mib) FROM chip_model WHERE NOT model_name IN (SELECT chip_model FROM phone)
SELECT AVG("ram_mib") FROM "chip_model" WHERE NOT "model_name" IN (SELECT "chip_model" FROM "phone")