Oracle extract()?????????????????????????????????????????Oracle extract??????????

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    select extract(year from date '2011-05-17') year from dual;
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    2011
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    select extract(month from date '2011-05-17') month from dual;
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    5
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    select extract(day from dt2-dt1) day,           extract(hour from dt2-dt1) hour,           extract(minute from dt2-dt1) minute,           extract(second from dt2-dt1) second    from (select to_timestamp('2011-02-04 15:07:00', 'yyyy-mm-dd hh24:mi:ss') dt1,           to_timestamp('2011-05-17 19:08:46', 'yyyy-mm-dd hh24:mi:ss') dt2    from dual)
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    102 4 1 46
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    select extract(year from systimestamp) year,           extract(month from systimestamp) month,           extract(day from systimestamp) day,           extract(minute from systimestamp) minute,           extract(second from systimestamp) second,           extract(timezone_hour from systimestamp) th,           extract(timezone_minute from systimestamp) tm,           extract(timezone_region from systimestamp) tr,           extract(timezone_abbr from systimestamp) ta    from dual;
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    2011 5 17 7 14.843 8 0 UNKNOWN UNK