I don't knw how but this solution worked for me!!!!! :) :)
strQuery = "SELECT Enquiry_Details.ProductId, Product_master.ProductCode, Enquiry_Header.UserId, Product_master.ProductShortName_EN, Enquiry_Details.Price, Enquiry_Details.Quantity, " +
" Enquiry_Header.EnquiryTotalAmount as [Total], Enquiry_Header.USD_Price" +
" FROM Enquiry_Details" +
" INNER JOIN Enquiry_Header ON Enquiry_Details.EnquiryID = Enquiry_Header.EnquiryID" +
" INNER JOIN Product_master ON Enquiry_Details.ProductID = Product_master.ProductId" +
" WHERE (Enquiry_Header.UserId = '"+ fuid+ "')";
OleDbDataAdapter adp = new OleDbDataAdapter(strQuery, con);
DataTable dt = new DataTable();
adp.Fill(dt);