Sql选择语句不工作WPF

我正在一个WPF工作，我有一个sql语句连接到数据库。该语句是搜索First_Quater，它做，但当我改变到Second_Quater(这不是在数据库中)，它会给我First_Quater的数字。我不能让它只在列中添加first_quarter。

我已经尝试了很多，搜索了互联网，但我仍然茫然。谢谢你的帮助。

SqlConnection con = new SqlConnection("Data Source=; Initial Catalog=; Integrated Security=True; Trusted_Connection=yes");
        con.Open();
        String comboquery = (@"SELECT * FROM [taxi_comm] WHERE First_Quarter = '" + checkedListBox1.SelectedItem + "'");
        SqlCommand cmd = new SqlCommand(comboquery, con);
        SqlDataReader dr = cmd.ExecuteReader();


            double sum = 0;
            for (int i = 0; i < gvDisplay.Rows.Count; ++i)
            {

                switch (checkedListBox1.SelectedItem.ToString().Trim())
                {
                    case "First Quarter":
                        foreach (string s in checkedListBox1.CheckedItems)
                        {
                            sum += Convert.ToInt32(gvDisplay.Rows[i].Cells[10].Value);
                            txtTotalGST.Text = sum.ToString(); 
                        }
                        MessageBox.Show("Its feb");
                        break;
                    case "Second Quarter":
                        foreach (string st in checkedListBox1.CheckedItems)
                        {
                            sum += Convert.ToInt32(gvDisplay.Rows[i].Cells[10].Value);
                            txtTotalGST.Text = sum.ToString();
                            MessageBox.Show("You have reached the second quarter");
                        }
                        break;