Question

我的SELECT有问题。当我在MS Access上执行它时，结果正确并完全返回。当我尝试通过C＃执行它时，相同的查询不返回任何内容。 SELECT的先前变体是通过适配器实现的，以在dataGridView中显示select的结果。但是当我执行它时，我只得到一个带有列名的空表。

private void getByObjectAndPollutant(string Object, string Pollutant, 
    int firstYear, int lastYear) 
{
    List<string> result = new List<string>();
    // line breaks added without concatenation, for readability.
    cmd = new OleDbCommand("SELECT object_name, p_name, mpc, emission_concentration, 
        [year] FROM Pollutants INNER JOIN (Objects INNER JOIN Emissions 
        ON Objects.o_id = Emissions.e_o_id) ON Pollutants.p_id = Emissions.e_p_id 
        WHERE (object_name = '" + Object + "' AND p_name = '" + Pollutant + "' AND 
        [year] BETWEEN " + firstYear + " AND " + lastYear + ") ;", con);          
    con.Open();
    reader = cmd.ExecuteReader();
    while (reader.Read()) {
        var myString = reader.GetString(0);
        result.Add(myString);
        richTextBox1.Text += myString;
    }
    con.Close();
}

这就是我在程序中称之为：

private void button5_Click(object sender, EventArgs e) {
    string localObject = comboBox5.Text.ToString();
    string localPollutant = comboBox4.Text.ToString();
    int localFirstYear = Convert.ToInt32(comboBox6.Text);
    int localLastYear = Convert.ToInt32(comboBox7.Text);
    getByObjectAndPollutant(localObject,localPollutant,localFirstYear,localLastYear);
}

这是MS Access中的SELECT查询：

SELECT object_name, p_name, mpc, emission_concentration, year  
FROM Pollutants 
INNER JOIN (Objects 
    INNER JOIN Emissions ON Objects.o_id=Emissions.e_o_id) 
ON Pollutants.p_id=Emissions.e_p_id  
WHERE object_name="some name" And p_name="some name" And year Between 2011 And 2015;

这是以前的方法代码：

private void getByObjectAndPollutant(string Object, string Pollutant, int firstYear, 
    int lastYear) 
{
    con.Open(); 
    DataTable dt = new DataTable(); 
    adapt = new OleDbDataAdapter(this long select); 
    adapt.Fill(dt); 
    dataGridView2.DataSource = dt; 
    con.Close();
}

我使用reader或adapter的主要原因是获取SELECT查询的结果。这个SELECT会返回什么吗？当这工作时，我将编码方法基于SELECT绘制图表。

Answer 1

可能是组合/下拉列表。奇怪的是，Toast.makeText(getApplicationContext(), ">" + data+"<",Toast.LENGTH_SHORT).show();返回选定的值，而不是文本。

尝试更改为

comboBox5.Text

Answer 2

您好！最后我找到了解决方案。 SELECT的主要目的是从DataBase获取一些数据，但是当我得到它时，我使用的不是ID   项目，我使用项目的名称，它不返回任何结果。所以我想让我   试着用ids做出条件。

所以工作函数的代码：

private void getByObjectAndPollutant(int Object, int Pollutant, int firstYear, int lastYear)
            {          
                con.Open();
                DataTable dt = new DataTable();
                adapt = new OleDbDataAdapter("SELECT mpc, emission_concentration FROM Pollutants INNER JOIN (Objects INNER JOIN Emissions ON Objects.o_id = Emissions.e_o_id) ON Pollutants.p_id = Emissions.e_p_id WHERE Objects.o_id=" + Object + " AND Pollutants.p_id=" + Pollutant + " AND Emissions.[year] BETWEEN " + firstYear + " AND " + lastYear + ";", con);
                adapt.Fill(dt);
                dataGridView2.DataSource = dt;
                con.Close();
            }

工作函数调用代码：

private void button5_Click(object sender, EventArgs e)
        {
            int localObject = Convert.ToInt32(comboBox4.SelectedValue.ToString());
            int localPollutant = Convert.ToInt32(comboBox5.SelectedValue.ToString());
            int localFirstYear = Convert.ToInt32(comboBox6.SelectedItem.ToString());
            int localLastYear = Convert.ToInt32(comboBox7.SelectedItem.ToString());

            getByObjectAndPollutant(localObject,localPollutant,localFirstYear,localLastYear);
        }

在结果DataGridView中，我得到了正确的数据，我非常高兴！感谢所有想要帮助我的人!!!

选择不返回结果

2 个答案: