What are the names, details and data types of the characteristics?
select characteristic_name, other_characteristic_details, characteristic_data_type from characteristics
Show these for the ones used in products.
select characteristic_name, other_characteristic_details, characteristic_data_type from characteristics as t1 join product_characteristics as t2 on t1.characteristic_id = t2.characteristic_id
Show the ones that are not used.
select characteristic_name, other_characteristic_details, characteristic_data_type from characteristics except select t1.characteristic_name, t1.other_characteristic_details, t1.characteristic_data_type from characteristics as t1 join product_characteristics as t2 on t1.characteristic_id = t2.characteristic_id