CREATE TABLE chip_model (RAM_MiB INT, model_name VARCHAR, chip_model VARCHAR)
CREATE TABLE phone (RAM_MiB INT, model_name VARCHAR, chip_model VARCHAR)

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")