Question

我正在尝试从C＃代码执行SQL查询，但它不返回任何值，但是当我直接向SQL编写查询时，它工作正常。

static int TestGettingData()
{
        int rows;
        string connectionString = System.Configuration.ConfigurationManager.ConnectionStrings["DefaultConnection"].ConnectionString;
        using (SqlConnection connection = new SqlConnection(connectionString))
        {
            using (SqlCommand command = new SqlCommand("SELECT [UserName] FROM [aspnet_Users]", connection))
            {
                connection.Open();
                rows = command.ExecuteNonQuery();
            }
        }
        return rows;
}

连接字符串：

<connectionStrings>
<add name ="DefaultConnection" connectionString="Server=myServer;
            Database=MyDatabase;User 
            Id=User;
            Password=password;
            MultipleActiveResultSets=True;"
            providerName="System.Data.SqlClient"/>
</connectionStrings>

TestGettingData()总是返回-1。

Answer 1

有多种方法可以从数据库中获取数据

<强>的ExecuteScalar

如果您有一个结果字段，则可以使用

string Command = "SELECT [UserName] FROM [aspnet_Users];";
using (SqlConnection myConnection = new SqlConnection(ConnectionString))
{
    myConnection.Open();
    using (SqlCommand myCommand = new SqlCommand(Command, myConnection))
    {
        string Result = (string)myCommand.ExecuteScalar(); // returns the first column of the first row
    }
}

SqlDataAdapter

如果您希望多行/列，可以通过

将其加载到DataTable中

string Command = "SELECT [UserName] FROM [aspnet_Users]";
using (SqlConnection myConnection = new SqlConnection(ConnectionString))
{
    using (SqlDataAdapter myDataAdapter = new SqlDataAdapter(Command, myConnection))
    {
        DataTable dtResult = new DataTable();
        myDataAdapter.Fill(dtResult);
    }
}

SqlDataReader

另一个解决方案是SQLDataReader。例如。如果要将列的所有行加载到List中。这比DataAdapter的开销要小。

List<string> Result = new List<string>();
string Command = "SELECT [UserName] FROM [aspnet_Users];";
using (SqlConnection mConnection = new SqlConnection(ConnectionString))
{
    mConnection.Open();
    using (SqlCommand cmd = new SqlCommand(Command, mConnection))
    {
        using (SqlDataReader reader = cmd.ExecuteReader())
        {
            while (reader.Read())
            {
                Result.Add((string)reader[0]);
            }
        }
    }
}

Answer 2

因为documentation这么说;

对于UPDATE，INSERT和DELETE语句，返回值为受命令影响的行数。当a上存在触发器时正在插入或更新的表，返回值包括数字受插入或更新操作影响的行数和数字受触发器或触发器影响的行数。 适用于所有其他类型的语句，返回值为-1 。如果发生回滚，则返回值也是-1。

您可以使用COUNT(*) ExecuteScalar方法获取您的电话号码。

using (SqlCommand command = new SqlCommand("SELECT COUNT([UserName]) FROM [aspnet_Users]", connection))
{
     connection.Open();
     rows = (int)command.ExecuteScalar();
}

Answer 3

我认为这是从数据库访问和检索数据的最佳方式：

创建DBconnection类

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace SQLexample
{
/// <summary>
/// A class that makes the connection to the SQL Database
/// </summary>
class DBconnection
{
    // variables
    private string sql_string;
    private string strCon;
    System.Data.SqlClient.SqlDataAdapter da_1;

    // set methods
    public string Sql
    {
        set { sql_string = value; }
    }

    public string connection_string
    {
        set { strCon = value; }
    }

    // DataSet
    public System.Data.DataSet GetConnection
    {
        get { return MyDataSet(); }
    }

    // MyDataSet method
    private System.Data.DataSet MyDataSet()
    {
        System.Data.SqlClient.SqlConnection con = new System.Data.SqlClient.SqlConnection(strCon);
        con.Open();

        da_1 = new System.Data.SqlClient.SqlDataAdapter(sql_string, con);

        System.Data.DataSet dat_set = new System.Data.DataSet();
        da_1.Fill(dat_set, "Table_Data_1");

        con.Close();

        return dat_set;
    }

    // Update DB method
    public void UpdateDB(System.Data.DataSet ds)
    {
        System.Data.SqlClient.SqlCommandBuilder cb = new System.Data.SqlClient.SqlCommandBuilder(da_1);
        cb.DataAdapter.Update(ds.Tables[0]);
    }
}
}

然后，在您的其他课程中，无论何时您想要访问数据库，都可以这样做：

try
{
    objConnect = new DBconnection();
    conStringAUTH = Properties.Settings.Default.authConnectionString;

    objConnect.connection_string = conStringAUTH;
    objConnect.Sql = "QUERY GOES HERE";

    ds = objConnect.GetConnection;

    // Data manipulation
    maxRows = ds.Tables[0].Rows.Count;

    if (maxRows == 0)
    {
        // Your query returned no values
    }

}
catch (Exception err)
{
    MessageBox.Show(err.Message);
}

希望这会有所帮助......

从C＃代码执行SQL查询

3 个答案: