Jasper report: two detail sections? - reporting

Question

I've got a jasper report to do, with data like this:
Item | Quantity | Color
------+-----------+--------
A001 | 1 | Red
A001 | 1 | Green
B002 | 3 | Red
B002 | 3 | Purple
The report is grouped by Item/Quantity, e.g.
Item: A001, Qty: 1, Colors: Red,Green
Item: B002, Qty: 3, Colors: Red,Purple
Now I've got this Jasper report that already groups as such - i.e. shows a heading with the item and quantity, with a list of colors underneath.
The question now is, underneath this group I need to display a number of horizontal lines (for someone to write something in), equal to the qty of the item. e.g. underneath the A001 group I need to display one line, and under the B002 group I need to display three lines, like so:
Item A001, Qty 1, Colors Red, Green
_________________________
Item B002, Qty 3, Colors Red, Purple
_________________________
_________________________
_________________________
I've tried looking at jasper scripts, but it seems they can only manipulate report parameters/variables.
Does anyone have an idea of how I could do this?

Answer 1

Hmm, interesting.
Here is what you could do:
Using this source data (MySQL):
create table items (
item varchar(4),
quantity number,
color varchar(10),
);
(insert data...)
create table numbers (i integer)
(insert data 0, 1, 2 .... MySQL 5.1 has stored procedures that could do it, earlier versions would need an external script to populate it. Go from 0 to the largest quantity you'd have).
Then, the trick is to craft the right sort of query. I came up with this:
select i.*, n.i from
(
select concat(i.item, ' ', i.quantity) as grouping, i.item, i.quantity,group_concat(distinct color) as colors
from items i
GROUP BY item, quantity
) i
cross join numbers n
where quantity > n.i;
E.g. If I populate my numbers table, and the populate the items table with your example data, and then run the query I get:
+----------+------+----------+------------+------+
| grouping | item | quantity | colors | i |
+----------+------+----------+------------+------+
| A001 1 | A001 | 1 | Red,Greem | 0 |
| B002 3 | B002 | 3 | Red,Purple | 0 |
| A001 1 | A001 | 1 | Red,Greem | 1 |
| B002 3 | B002 | 3 | Red,Purple | 1 |
| A001 1 | A001 | 1 | Red,Greem | 2 |
| B002 3 | B002 | 3 | Red,Purple | 2 |
| A001 1 | A001 | 1 | Red,Greem | 3 |
| B002 3 | B002 | 3 | Red,Purple | 3 |
+----------+------+----------+------------+------+
Then in your Jasper Report, the trick is to create a group/band that works off the 'grouping' column, and put your heading in that:
Item A001, Qty 1, Colors Red, Green
And then, in the detail section just have a line as the only thing in the detail.
Doing this generates the report you want for me.
Note that the numbers table is a little silly, but is a standard data warehousing technique, though I suspect some database (e.g. Oracle) would have clever recursive procedures or other functions that would exclude the need for it.

Answer 2

OK, in the end, this was done using a subdataset and a crosstab in the detail footer section. Works nicely - thanks for everyone who contributed :)