必须声明标量变量“@id”

这串代码有问题吗
string id = Request.QueryString["id"];
string sql = "select * from BookInfo where id =@id";
DataTable table = DAB.ExecuteDataTable(sql);
SqlConnection con = new SqlConnection();
SqlCommand cmd = new SqlCommand(sql, con);
cmd.Parameters.Add(new SqlParameter("@id", SqlDbType.Int)).Value = Request.QueryString["id"];
cmd.ExecuteNonQuery();

       cmd.Parameters.Add(new SqlParameter("@id", SqlDbType.Int)).Value = Request.QueryString["id"];

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string id = Request.QueryString["id"];
string sql = "select * from BookInfo where id = @id";
SqlConnection con = new SqlConnection();
SqlCommand cmd = new SqlCommand(sql, con);
cmd.Parameters.AddWithValue("@id", id);
SqlDataReader reader = cmd.ExecuteReader();