返回选择语句的值

本文关键字：语句 选择 返回 | 更新日期: 2023-09-27 17:49:38

我想将选择语句的结果值检索到一个字符串变量中。这样的:

OleDbCommand cmd1 = new OleDbCommand();
cmd1.Connection = GetConnection();
cmd1.CommandText = "SELECT treatment FROM appointment WHERE patientid = " + text;
cmd1.ExecuteNonQuery();

我想将选择的处理值放入字符串变量中。我该怎么做呢?

返回选择语句的值

使用ExecuteReader()而不是ExecuteNonQuery()。ExecuteNonQuery()只返回受影响的行数。

try
{
    SqlDataReader dr = cmd1.ExecuteReader();
}
catch (SqlException oError)
{
}
while(dr.Read())
{
    string treatment = dr[0].ToString();
}

或者使用using语句更好。

using(SqlDataReader dr = cmd1.ExecuteReader())
{
    while(dr.Read())
    {
        string treatment = dr[0].ToString();
    }
}

但是如果你的SqlCommand只返回1列，你可以使用ExecuteScalar()方法。返回第一行的第一列，如下所示:-

cmd.CommandText = "SELECT treatment FROM appointment WHERE patientid = " + text;
string str = Convert.ToString(cmd.ExecuteScalar());

你也可以打开你的代码SQL注入。始终使用参数化查询。Jeff有一篇很酷的博客文章叫《要么给我参数化SQL，要么给我死亡》。请仔细阅读。另请阅读DotNetPerl SqlParameter文章。

Execute Scalar: Getting Single Value from Database方法从数据库中检索单个值(例如，聚合值)。

cmd1.Connection = GetConnection();
cmd1.CommandText = "SELECT treatment FROM appointment WHERE patientid = " + text;
if(cmd.ExecuteScalar()==null)
{
    var treatment = cmd.ExecuteScalar();
}
其他方式:ExecuteReader() 
try
{
    cmd1.CommandText ="SELECT treatment FROM appointment WHERE patientid=@patientID";
    cmd1.Parameters.AddWithValue("@patientID", this.DropDownList1.SelectedValue);
    conn.Open();
    SqlDataReader dr = cmd1.ExecuteReader();
    while (dr.Read())
    {
        int PatientID = int.Parse(dr["treatment"]);
    }
    reader.Close();
    ((IDisposable)reader).Dispose();//always good idea to do proper cleanup
}
catch (Exception exc)
{
    Response.Write(exc.ToString());
}

答案:

String res = cmd1.ExecuteScalar();

注释:使用参数化查询来防止SQL注入

你的示例代码有很多错误。

  1. 你有内联sql，这打开了sql注入的主要方式。

  2. 你正在使用ExecuteNonQuery()，这意味着你没有得到任何数据。

     string sSQL = "SELECT treatment FROM appointment WHERE patientid = @patientId";
 OleDbCommand cmd1 = new OleDbCommand(sSQL, GetConnection()); // This may be slight different based on what `GetConnectionReturns`, just put the connection string in the second parameter.

    cmd1.Parameters.AddWithValue("@patientId", text);
    SqlDataReader reader = cmd1.ExecuteReader();
    string returnValue;
    while(reader.Read())
    {
       returnValue = reader[0].ToString();
    }

您只需要使用命令的ExecuteScalar方法—这将为您提供结果集的第一行和第一列的值。

OleDbCommand cmd1 = new OleDbCommand();
cmd1.Connection = GetConnection();
cmd1.CommandText = "SELECT treatment FROM appointment WHERE patientid = " + text;
var result = cmd1.ExecuteScalar();

如果您的SQL语句返回多个行/列，那么您可以使用ExecuteReader()

您需要使用OleDbAdapter。

string connection = "your connection";
string query = "SELECT treatment FROM appointment WHERE patientid = " + text;
OleDbConnection conn = new OleDbConnection(connection);
OleDbDataAdapter adapter = new OleDbDataAdapter();
adapter.SelectCommand = new OleDbCommand(query, conn);
adapter.Fill(dataset);

SqlConnection dbConnect = new SqlConnection("your SQL connection string");    
string name = " 'ProjectName' ";
string strPrj = "Select e.type, (e.surname +' '+ e.name) as fulln from dbo.tblEmployees e where id_prj = " + name;
        SqlCommand sqlcmd = new SqlCommand(strPrj, dbConnect);
        SqlDataAdapter sda = new SqlDataAdapter(strPrj, dbConnect);
        ds = new DataSet();
        sda.Fill(ds);
        dbConnect.Open();
        sqlcmd.ExecuteNonQuery();
        dbConnect.Close();
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