SQL-Problem mit Checkout-Zeiten

Ich vermute, Sie möchten den Unterschied zwischen der minimalen Check-in-Zeit (wobei checktype =1) und der maximalen Checkout-Zeit (wobei checktype =0) finden.

select userID,
    min_date,
    max_date,
    (max_date - min_date) diff
from (
    select distinct userID,
        (
            select min(checktime) 
            from checkinout t2
            where t1.userID = t2.userID
                and t2.checktype = 1
        ) min_date,
        (
            select max(checktime) 
            from checkinout t3
            where t1.userID = t3.userID
                and t3.checktype = 0
        ) max_date
    from checkinout t1
    )
order by userID