ADO中SqlCommand的三种执行

1、ExecuteNonQuery（）//返回受影响行数，如果多条就会累加 

string connStr = @"server=WIN-20161229QGK\SQLEXPRESS;uid=PJ;pwd=123;database=MySchoolMoreData";
            using (SqlConnection conn=new SqlConnection (connStr))
            {
                conn.Open();
                string sql = string.Format("insert into grade values('{0}');select count(*)from grade",textBox1.Text.Trim ());
                SqlCommand command = new SqlCommand(sql,conn);
                int num = (int)command.ExecuteNonQuery();
                MessageBox.Show(num.ToString());
            }

2、ExecuteScalar（）//返回查询结果的第一行第一列的值

string connStr = @"server=WIN-20161229QGK\SQLEXPRESS;uid=PJ;pwd=123;database=MySchoolMoreData";
            using (SqlConnection conn=new SqlConnection (connStr))
            {
                conn.Open();
                string sql = string.Format("insert into grade values('{0}');select count(*)from grade",textBox1.Text.Trim ());
                SqlCommand command = new SqlCommand(sql,conn);
                int num = (int)command.ExecuteScalar();
                MessageBox.Show(num.ToString());
            }

3、ExecuteReader（）//读取数据表，并存入缓存，就是数据读取器。把数据一条条的读出

string connStr = @"server=WIN-20161229QGK\SQLEXPRESS;uid=PJ;pwd=123;database=MySchoolMoreData";
            using (SqlConnection conn=new SqlConnection (connStr))
            {
                conn.Open();
                string sql = "select classid,classname from grade";
                SqlCommand command = new SqlCommand(sql,conn);
                SqlDataReader reader = command.ExecuteReader();
                while (reader.Read())//判断有没有下一个数据，类似于迭代器
                {
                     ListViewItem lv = new ListViewItem(reader["classid"].ToString());
                    lv.SubItems.Add(reader["classname"].ToString());
                    listView1.Items.Add(lv);
                }
            }