附近有语法错误，该如何解决？

string sql = "update BookInfo set sl = sl-1 where id = " + Request.QueryString["id"];
//error
sql = "insert into Borrow(U_code,U_name,B_id,B_day,B_state) values ('" + Session["Login_code"].ToString() + "','" + Session["Login_name"].ToString() + "','" + Request.QueryString["id"] + "','" + B_day.ToString() + "',N'借阅中')";
DAB.ExecuteNonQuery(sql);

你的where id =的地方需要加引号