Oracle EXTRACT()函数与to_char() 函数

梦里梦外; 2022-07-12 13:45 153阅读 0赞

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')      完毕!`