select to_timestamp(datetime,  "yyyy-MM-dd hh:mm:ss.SSS") from
(
    select nvl(to_timestamp(datetime, "yyyy/MM/dd' 'HH:mm:ss.SSS"),to_timestamp(datetime, "dd-MM-yyyy' 'HH:mm:ss.SSS")) as datetime
    from
    (
     select "2017/08/01 02:25:59.000" as datetime
     union all
     select "01-08-2017 02:30:59.000" as datetime 
    ) a
) b;

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