原文链接：http://www.cnblogs.com/oradragon/archive/2012/08/30/2664674.html

Syntax

 

extract_datetime::=

Purpose

EXTRACT extracts and returns the value of a specified datetime field from a datetime or interval expression. The expr can be any expression that evaluates to a datetime or interval data type compatible with the requested field:

• If YEAR or MONTH is requested, then expr must evaluate to an expression of data type DATE, TIMESTAMP, TIMESTAMP WITH TIME ZONE, TIMESTAMP WITH LOCAL TIME ZONE, or INTERVAL YEAR TO MONTH.

• If DAY is requested, then expr must evaluate to an expression of data type DATE, TIMESTAMP, TIMESTAMP WITH TIME ZONE, TIMESTAMP WITH LOCAL TIME ZONE, or INTERVAL DAY TO SECOND.

• If HOUR, MINUTE, or SECOND is requested, then expr must evaluate to an expression of data type TIMESTAMP, TIMESTAMP WITH TIME ZONE, TIMESTAMP WITH LOCAL TIME ZONE, or INTERVAL DAY TO SECOND. DATE is not valid here, because Oracle Database treats it as ANSI DATE data type, which has no time fields.

• If TIMEZONE_HOUR, TIMEZONE_MINUTE, TIMEZONE_ABBR, TIMEZONE_REGION, or TIMEZONE_OFFSET is requested, then expr must evaluate to an expression of data type TIMESTAMP WITH TIME ZONE or TIMESTAMP WITH LOCAL TIME ZONE.

EXTRACT interprets expr as an ANSI datetime data type. For example, EXTRACT treats DATE not as legacy Oracle DATE but as ANSI DATE, without time elements. Therefore, 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 data type.

When you specify TIMEZONE_REGION or TIMEZONE_ABBR (abbreviation), the value returned is a VARCHAR2 string containing the appropriate time zone region name or abbreviation. When you specify any of the other datetime fields, the value returned is an integer value of NUMBER data type representing the datetime value in the Gregorian calendar. When extracting from a datetime with a time zone value, the value returned is in UTC. For a listing of time zone region names and their corresponding abbreviations, query the V$TIMEZONE_NAMES dynamic performance view.

1. 获取当前日期中的年份

SQL> select extract(year from sysdate) as "year" from dual
  2  /

      year
----------
      2012

 2. 获取当前日期中的月份

SQL> select extract(month from sysdate) as "month" from dual
  2  /

     month
----------
         8

 3. 获取当前日期中的日 

SQL> select extract(day from date'2012-12-28') as "day" from dual
  2  /

       day
----------
        28

 

转载于:https://www.cnblogs.com/oradragon/archive/2012/08/30/2664674.html