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.

• 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;