Oracle EXTRACT()函数
EXTRACT extracts and returns the value of a specified datetime field from a datetime or interval value expression.
The field you are extracting must be a field of the datetime_value_expr or interval_value_expr. For example, you can extract only YEAR, MONTH, and DAY from a DATE value. Likewise, you can extract TIMEZONE_HOUR and TIMEZONE_MINUTE only from the TIMESTAMP WITH TIME ZONE datatype.
eg.
SQL> select sysdate from dual;
/
select extract(year from sysdate) year from dual;
/
select extract(month from sysdate) month from dual;
/
select extract(day from sysdate) day from dual;
/
select extract(hour from timestamp '2010-11-18 17:57:32') hour from dual;
/
select extract(minute from timestamp '2010-11-18 17:57:32') minute from dual;
/
--but not
select extract(hour from sysdate) from dual;
/
select extract(minute from sysdate) from dual;
/
SYSDATE
---------
18-NOV-10
SQL>
SYSDATE
---------
18-NOV-10
SQL>
YEAR
----------
2010
SQL>
YEAR
----------
2010
SQL>
MONTH
----------
11
SQL>
MONTH
----------
11
SQL>
DAY
----------
18
SQL>
DAY
----------
18
SQL>
HOUR
----------
17
SQL>
HOUR
----------
17
SQL>
MINUTE
----------
57
SQL>
MINUTE
----------
57
SQL> SQL> SQL> select extract(hour from sysdate) from dual
*
ERROR at line 1:
ORA-30076: invalid extract field for extract source
SQL> select extract(hour from sysdate) from dual
*
ERROR at line 1:
ORA-30076: invalid extract field for extract source
SQL> select extract(minute from sysdate) from dual
*
ERROR at line 1:
ORA-30076: invalid extract field for extract source
SQL>
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