if exists query

what is the problem in this code

i have erorr in this line  (

string query = "if exists(select Avatar from pets  Where Skey = @Skey);

)

      string connectionString = WebConfigurationManager.ConnectionStrings["conString"].ConnectionString;
           SqlConnection con = new SqlConnection(connectionString);
           string query = "if exists(select Avatar from pets  Where Skey = @Skey);
           query += " begin select Avatar from pets  Where Skey = @Skey end";
           query += " else  begin select Avatar from pets  Where Skey = '1' end" ;
           SqlCommand cmd = new SqlCommand(query, con);
           con.Open();
           cmd.Parameters.AddWithValue("@Skey", Request.QueryString["ID"]);
           object ReturnImage = cmd.ExecuteScalar();
           byte[] photo = (byte[])ReturnImage;
           Response.BinaryWrite(photo);
           con.Close();