Table schema:
CREATE TABLE chip_model (RAM_MiB INTEGER, model_name VARCHAR, chip_model VARCHAR); CREATE TABLE phone (RAM_MiB INTEGER, model_name VARCHAR, chip_model VARCHAR)

Question: 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)