SQL Query for Chart does not work

Question

Have written a query that works fine outside of the app, but no data returned (displayed) within the app. I suspect i could simply create a view with the query in and all would be fine, but i'm intrigued to know why it will not yield data as is. Any ideas? Here's the query:
select * from
select null link, period, data from (
select dp3.period, dp3.data-dp14.data data from
(select period, data from ml_data_daily
where data_point = 3
and category = 'A. 0'
and period_type = 'DAY'
and period >= (select period from ml_period where period_number = :p1_date_start)
and period <= (select period from ml_period where period_number = :p2_date_end)
) dp3,
(select period, data from ml_data_daily
where data_point = 14
and category = 'Invalid Search'
and period_type = 'DAY'
and period >= (select period from ml_period where period_number = :p1_date_start)
and period <= (select period from ml_period where period_number = :p2_date_end)
) dp14
where dp3.period=dp14.period
order by period)
)

Answer 1

Bruce - Make sure the items accessed via the bind variables have been set in session state. If those are just search fields, the chart query won't see them until the page has been submitted.
Scott