Oracle EXTRACT()函數與to_char() 函數

轉自：http://www.cnblogs.com/ruanbl/archive/2008/02/29/1086398.html

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.

• SELECT EXTRACT(YEAR FROM SYSDATE) FROM DUAL; return Current Year
• SELECT EXTRACT(MONTH FROM SYSDATE) FROM DUAL; return Current Month
• SELECT EXTRACT(DAY FROM SYSDATE) FROM DUAL; return Current Day
• SELECT EXTRACT(HOUR FROM SYSDATE) FROM DUAL;Error

       Correct:SELECT EXTRACT(HOUR FROM TIMESTAMP '2005-10-10 10:10:10') FROM DUAL;


• SELECT EXTRACT(MINUTE FROM SYSDATE) FROM DUAL;Error

       Correct:SELECT EXTRACT(MINUTE FROM TIMESTAMP '2005-10-10 10:10:10') FROM DUAL;

       以上是EXTRACT()函數的使用說明,英文部分有個很重要的部分.就是extract 抽取年月日部分和時分部分的使用方法是不同的.
EXTRACT(YEAR FROM SYSDATE)中FROM 的下一個參數是日期類型的時候可以抽取年月日.就是說這個參數是日期類型的時候就可以抽取年\月\日,而抽取不了時\分部分.抽取時分部分得在FROM 後面加帶TIMESTAMP 且後面加時間字元.
   這裡要注意,在(MINUTE FROM TIMESTAMP '2005-10-10 10:10:10')中,如果把'2005-10-10 10:10:10'用日期類型的欄位來代替並轉換成字元類型後,也是錯誤的,估計是這方法的問題.
    比如(MINUTE FROM TIMESTAMP to_date(to_char(日期類型,'YYYY-MM-DD hh24:mi:ss'),'YYYY-MM-DD hh24:mi:ss')),這樣執行不了的.

    如果真要從日期類型的欄位中抽取時\分部分用to_char() 函數就可以了.比如to_char(日期類型,'hh24:mi') 

    完畢!