大一期末考试类型

 public class DBHelper
    {
        //创建连接字符串
        public static string str = "server=.;database=StuDB;Integrated Security=true";
        public static SqlConnection conn = null;
        //增删改
        //查询
        public static SqlDataReader GetDataReader(string sql)
        {
            conn = new SqlConnection(str);
            conn.Open();
            SqlCommand comm = new SqlCommand(sql, conn);
            SqlDataReader reader = comm.ExecuteReader();
            return comm.ExecuteReader(CommandBehavior.CloseConnection);
        }

        //增删改
        public static Boolean QuNoerey(string sql)
        {
            conn = new SqlConnection(str);
            conn.Open();
            SqlCommand comm = new SqlCommand(sql, conn);
            int num = comm.ExecuteNonQuery();
            conn.Close();//关闭数据库链接
            return num > 0;
        }

        //填充 查询
        public static DataTable Query(string sql)
        {
            DataTable ds = new DataTable();
            conn = new SqlConnection(str);
            SqlDataAdapter reader = new SqlDataAdapter(sql, conn);
            reader.Fill(ds);
            return ds;
        }
        //聚合函数
        public static object scalar(string sql)
        {
            conn = new SqlConnection(str);
            conn.Open();
            SqlCommand comm = new SqlCommand(sql, conn);
            object obj = comm.ExecuteScalar();
            return obj;
        }
    }