Use this query instead:
SELECT a.DoDate,
a.DoNumber,
a.CusId,
b.CompName,
b.CompAddress,
b.Tel,b.Fax,
b.ContPerson,
b.SalesPerson,
c.ItemNo,
c.Description,
c.Qty
from ItemParent
INNER JOIN CustomerDetail b ON a.CusId = b.CusId
INNER JOIN ItemParent c ON b.DoNumber = c.DoNumber
Regards,
Eduard