Some questions about SQL group by week - mysql

Question

I have some problems when coding SQL group by week.
I have a MySQL table named order.
In this entity, there are several attributes, called 'order_id', 'order_date', 'amount', etc.
I want to make a table to show the statistics of past 7 days order sales amount.
I think first I should get the today value.
Since I use Java Server Page, the code like this:
Calendar cal = Calendar.getInstance();
int day = cal.get(Calendar.DATE);
int Month = cal.get(Calendar.MONTH) + 1;
int year = cal.get(Calendar.YEAR);
String today = year + "-" + Month + "-" + day;
then, I need to use group by statement to calculate the SUM of past 7 day total sales amount.
like this:
ResultSet rs=statement.executeQuery("select order_date, SUM(amount) " +
"from `testing`.`order` GROUP BY order_date");
I have problem here. In my SQL, all order_date will be displayed.
How can I modify this SQL so that only display past seven days order sale amount?
Besides that, I discover a problem in my original SQL.
That is, if there is no sales on that day, no results would be displayed.
OF course, I know the ResultSet does not allow return null values in my SQL.
I just want to know if I need the past 7 order sales even the amount is 0 dollars,
Can I have other methods to show the 0?
Please kindly give me advices if you have idea.
Thank you.

Answer 1

Usually it occurs to create with a script or with a stored procedure a calendar table with all dates.
However if you prefer you can create a table with few dates (in your case dates of last week) with a single query.
This is an example:
create table orders(
id int not null auto_increment primary key,
dorder date,
amount int
) engine = myisam;
insert into orders (dorder,amount)
values (curdate(),100),
(curdate(),200),
('2011-02-24',50),
('2011-02-24',150),
('2011-02-22',10),
('2011-02-22',20),
('2011-02-22',30),
('2011-02-22',5),
('2011-02-19',10);
select t.cdate,sum(coalesce(o.amount,0)) as total
from (
select curdate() -
interval tmp.digit * 1 day as `cdate`
from (
select 0 as digit union all
select 1 union all
select 2 union all
select 3 union all
select 4 union all
select 5 union all
select 6 union all
select 7 ) as tmp) as t
left join orders as o
on t.cdate = o.dorder and o.dorder >= curdate() - interval 7 day
group by t.cdate
order by t.cdate desc
Hope that it helps. Regards.

Answer 2

To answer your question "How can I modify this SQL so that only display past seven days order sale amount?"
Modify the SQL statement by adding a where clause to it:
Where order_date >= #date_7days_ago
The value for this #date_7days_ago date variable can be set before your statement:
Select #date_7days_ago = dateadd(dd,-7,getdate())
Adding that where clause to your query will return only those records which order date is in the last seven days.
Hope this helps.

Answer 3

You can try using this:
ResultSet rs = statement.executeQuery(
"SELECT IFNULL(SUM(amount),0)
FROM table `testing`.`order`
WHERE order_date >= DATE_SUB('" + today + "', INTERVAL 7 DAY)"
);
This will get you the number of orders made in the last 7 days, and 0 if there were none.