從 oracle 數據庫中檢索 C# 中的值

Question

我不知道為什么，但嘗試從 oracle 數據庫中的表中獲取最大 integer 值時遇到錯誤：這是代碼：

string oradb = "Data Source=";
            oradb = oradb + Login.db + ";";
            oradb = oradb + "User Id=" + Login.user;
            oradb = oradb + ";Password=" + Login.pass + ";";
            OracleConnection conn = new OracleConnection(oradb);
            conn.Open();
            string term = GetTerminal(terminal);
            string sql = "SELECT tallynumber from "+frmSchemas.schema + ".tallies" ;
             MessageBox.Show(sql);
            OracleCommand cmd = new OracleCommand(sql, conn);
            cmd.CommandType = CommandType.Text;
            OracleDataReader dr = cmd.ExecuteReader();
            MessageBox.Show("1");
            Int64 TallyNo = dr.GetInt32(0);  



      // lblTallyNo.Text = (TallyNo).ToString()

返回值應該是： 72332 如果有任何相關性

Answer 1

如果你想從一個table中找到max value ， query應該是

SELECT max(tallynumber) from ....

如果你想獲得max, min etc等值，你應該調用ExecuteScalar()而不是ExecuteReader().

所以你的代碼可以變成

   string sql = "SELECT Max(tallynumber) from "+frmSchemas.schema + ".tallies";              MessageBox.Show(sql);             
    OracleCommand cmd = new OracleCommand(sql, conn);             
    cmd.CommandType = CommandType.Text;             
    object val = cmd.ExecuteScalar();
int res = int.MinValue;             
    //MessageBox.Show("1");
  if(int.TryParse(val.ToString(), out res)) 
    Int64 TallyNo = res;

Answer 2

您缺少dr.Read(); 在訪問值之前：

OracleDataReader dr = cmd.ExecuteReader();
dr.Read();
Int64 TallyNo = dr.GetInt32(0);

Answer 3

您可以嘗試以下

        string sql = "SELECT max(tallynumber) from "+frmSchemas.schema + ".tallies" ;
         MessageBox.Show(sql);
        OracleCommand cmd = new OracleCommand(sql, conn);
        cmd.CommandType = CommandType.Text;
        OracleDataReader dr = cmd.ExecuteReader();
        MessageBox.Show("1");
        if (dr.Read())
        {
        Int64 TallyNo = dr.GetInt32(0); 
        }

編輯：

if(dr.Read())
{
Int64 TallyNo = Convert.ToInt64(dr["tallynumber"].ToString());
}