使用组合框将列表视图与SQL表组合起来

我正试图获得一个组合框来在列表视图中填充技术的发票，但到目前为止还不起作用。

这是我在SQL server中的表：

Tech

Tech_Id (PK) | First | Last | Phone | Customer_Count | --------------+-------+------+-------+----------------+ 1 | Tim | Bo |9384027| 7 | 2 | Andy | Wong |9374927| 8 | 3 | Jack | Help |2183847| 8 |

Invoices

Invoice_Id (PK) | Job_Descr |Job_Date | Total | Term | Customer_Id | Tech_Id | -----------------+-------------+---------+-------+------+-------------+---------+ 1 |Changed Motor|11/3/2014| $30 | 30 | 2 | 1 | 2 |Tune up |4/15/2013| $50 | 30 | 11 | 2 | 3 |Changed PS |9/14/2014| $100 | 30 | 8 | 1 |

这是我在c#中的代码：

    public TechForm()
    {
        InitializeComponent();
        SqlDataAdapter da = new SqlDataAdapter("SELECT (First+' '+Last) AS Name FROM Tech Order By First", con);
        con.Open();
        DataTable dt = new DataTable();
        da.Fill(dt);
        for (int i = 0; i < dt.Rows.Count; i++)
        {
            comboBox1.Items.Add(dt.Rows[i]["Name"]);
        }
        con.Close();
    }

    private void button1_Click(object sender, EventArgs e)
    {
        con.Open();
        SqlCommand cmd = new SqlCommand("INSERT INTO Tech (First, Last, Phone) VALUES ('" + textBox2.Text + "', '" + textBox3.Text + "', '" + textBox4.Text + "')", con);
        cmd.ExecuteNonQuery();
        con.Close();
    }
    private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
    {
        listView1.Items.Clear();
        con.Open();
        SqlCommand cm = new SqlCommand("SELECT * FROM Invoices JOIN Tech ON Invoices.Tech_Id = Tech.Tech_Id WHERE Last='" + comboBox1.Text + "' ; ", con);

            try
            {
                SqlDataReader dr = cm.ExecuteReader();
                while (dr.Read())
                {
                    ListViewItem item = new ListViewItem(dr["Invoice_Id"].ToString());
                    item.SubItems.Add(dr["Job_Descr"].ToString());
                    item.SubItems.Add(dr["Job_Date"].ToString());
                    item.SubItems.Add(dr["Total"].ToString());
                    item.SubItems.Add(dr["Customer_Id"].ToString());
                    listView1.Items.Add(item);
                }

            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message, Application.ProductName, MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
            con.Close();
    }