DATEDIFF

SELECT HISTORY_ID, CAR_ID, 
        date_format(START_DATE, '%Y-%m-%d') START_DATE, 
        date_format(END_DATE,'%Y-%m-%d') END_DATE,
        case when datediff(END_DATE, START_DATE)+1 >= 30 then '장기 대여'
        else '단기 대여'
        end RENT_TYPE
from CAR_RENTAL_COMPANY_RENTAL_HISTORY
where month(start_date)=9
order by HISTORY_ID desc

대여 기간 30일 이상