Oracle extract函数抽取时间

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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()函数的使用说明,英文部分有个很重要的部分.就是extract 抽取年月日部分和时分部分的使用方法是不同的.
EXTRACT(YEAR FROM SYSDATE)中FROM 的下一个参数是日期类型的时候可以抽取年月日.就是说这个参数是日期类型的时候就可以抽取年\月\日,而抽取不了时\分部分.抽取时分部分得在FROM 后面加带TIMESTAMP 且后面加时间字符.
这里要注意,在(MINUTE FROM TIMESTAMP '2005-10-10 10:10:10')中,如果把'2005-10-10 10:10:10'用日期类型的字段来代替并转换成字符类型后,也是错误的,估计是这方法的问题.
比如(MINUTE FROM TIMESTAMP to_date(to_char(日期类型,'YYYY-MM-DD hh24:mi:ss'),'YYYY-MM-DD hh24:mi:ss')),这样执行不了的.

如果真要从日期类型的字段中抽取时\分部分用to_char() 函数就可以了.比如to_char(日期类型,'hh24:mi')
转自：http://www.cnblogs.com/ruanbl/archive/2008/02/29/1086398.html
　　
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