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