Show me assets each maintenance contract contains.
select * from maintenance_contracts as t1 join assets as t2 on t1.maintenance_contract_id = t2.maintenance_contract_id group by t1.maintenance_contract_id
Show me the number of assets and contract id.
select count(*), t1.maintenance_contract_id from maintenance_contracts as t1 join assets as t2 on t1.maintenance_contract_id = t2.maintenance_contract_id group by t1.maintenance_contract_id