将日期与日期时间格式进行比较

Question

How do I shorten the following statement:

select * from orders
where to_char(trunc(Cancel_Date,'MONTH'),'dd/mm/yyyy')='01/03/2015'
  and state = 'Cancelled'
  and to_date(to_char(trunc(Order_Date,'MONTH'),'dd/mm/yyyy'),'dd/mm/yyyy') < to_date('01/03/2015','dd/mm/yyyy')

Answer 1

It looks like you're trying to retrieve orders that were ordered before March but cancelled in March? If so, I would avoid filters like this: to_char(trunc(Cancel_Date,'MONTH'),'dd/mm/yyyy')='01/03/2015' ; if you have an index on cancel_date , then it won't be used (except on the off chance that you have a function-based index on that column!). I would do the following:

SELECT * FROM orders
 WHERE status = 'Cancelled'
   AND cancel_date >= DATE'2015-03-01'
   AND cancel_date < DATE'2015-04-01'
   AND order_date < DATE'2015-03-01';

In the above query I am using ANSI date literals (supported in Oracle since 9i, I believe) rather than using TO_CHAR() , TO_DATE() , etc. And I am not applying any functions to cancel_date or order_date so that the optimizer can use the indexes on those columns (if they exist).

On a side note using SELECT * rather than explicitly naming the columns you need is generally not considered a best practice.

Answer 2

in the second part of the where condition truncate isn't necessary:

select * 
from orders
where to_char(trunc(Cancel_Date,'MONTH'),'dd/mm/yyyy')='01/03/2015'
  and state = 'Cancelled'
  and Order_Date < to_date('01/03/2015','dd/mm/yyyy')

Answer 3

The function trunc returns a date. You don't need to convert that to char and back to date again.

where trunc(Cancel_Date,'MONTH') = to_date('01/03/2015','dd/mm/yyyy')
and state = 'Cancelled'
and trunc(Order_Date,'MONTH') < to_date('01/03/2015','dd/mm/yyyy')