MySql重叠日期无法正确完成

I am trying to make a query with overlapping dates. My tabel look like this'

Agenda ID   DateStart               DateEnd
-------------------------------------------------------
1           2012-04-02 08:00:00     2012-04-02 11:30:00
2           2012-04-02 10:00:00     2012-04-02 13:00:00
3           2012-04-02 12:00:00     2012-04-02 15:00:00

This data is for planning activities. You can't plan a activity if the time overlaps. In this case all the times overlap. But he compares it to the one you planned earlier. Say I planned ID 1, so I can't plan ID 2, But I can plan ID 3.

How Can I write A query with php to select the items that overlap. Thank you very much

SELECT
   agenda.DateStart,
   agenda.DateEnd,
   agenda.AgendaID
FROM
   AGENDA
WHERE
   ????
   AND
   ????

select a.DateStart, a.DateEnd, a.AgendaId, b.DateStart, b.DateEnd, b.AgendaId
from agenda a, agenda b
where 
a.AgendaId <> b.AgendaId
and b.DateStart>= a.DateStart and
b.DateStart<= a.DateEnd

$query_Orders = "SELECT * FROM AGENDA WHERE fldTime between  DateStart  and DateENd ";

You can check with the MYSQL BETWEEN keyword.