sql - Retrieving Online Users By Login and Logout Records -

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consider userlog table above. save login or logout operation of users table. how can retrieve online users , login time?

something this:

select * userlog l1 operation = 'enter' ,        not exists(select * userlog l2                   l1.user = l2.user ,                         l2.operation = 'exit' ,                         l2.time > l1.time)