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

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")