CREATE TABLE chip_model (RAM_MiB INTEGER, model_name VARCHAR(255), chip_model VARCHAR(255)); CREATE TABLE phone (RAM_MiB INTEGER, model_name VARCHAR(255), chip_model VARCHAR(255))
Find the average ram mib size of the chip models that are never used by any phone.
    
SQL: SELECT AVG(RAM_MiB) FROM chip_model WHERE NOT model_name IN (SELECT chip_model FROM phone)