you can use the following code in a procedure, get the result and bind it to your gridview
select Name,Age, DOB, "Contact No"=
CASE
WHEN landphone IS NOT NULL THEN (SELECT landphone from customer cst where cst.CustID = outCust.CustID)
WHEN mobilephone IS NOT NULL THEN (SELECT mobilephone from customer cst where cst.CustID = outCust.CustID)
WHEN officeno IS NOT NULL THEN (SELECT officeno from customer cst where cst.CustID = outCust.CustID)
WHEN fax IS NOT NULL THEN ( SELECT fax from customer cst where cst.CustID = outCust.CustID)
ELSE 'no contact no'
END
FROM Customer outCust
I've used a table something like your table definition (only the difference is I've one Primary Key CustID)