oracle "with as" "extract"
with a as (select * from test)select * from a;其实就是把一大堆重复用到的SQL语句放在with as 里面，取一个别名，后面的查询就可以用它这样对于大批量的SQL语句起到一个优化的作用，而且清楚明了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 YearSELECT EXTRACT(MONTH FROM SYSDATE) FROM DUAL; return Current MonthSELECT EXTRACT(DAY FROM SYSDATE) FROM DUAL; return Current DaySELECT 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;
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