Create adminpage in ASP.Net

In my website project ASP.Net, i use a SqlConnection and SqlAdapter query data in Sql server 2008. I have report a variable is : maddh = txtddh.Text.ToString() and
 SqlDataAdapter da = new SqlDataAdapter("select SanPham.TenSP,CTDDH.SoLuong,CTDDH.Gia from SanPham,CTDDH,DDH where CTDDH.MaDDH= DDH.MaDDH and CTDDH.MaSP=SanPham.MaSP and DDH.MaDDH = " + maddh , con);
Running my page, failed " Incorrect syntax near '='.". I think my project false in join sequence and variable. How i can join true??Help me.