Почему в коде в переменную при запросе с помощью sql не получает требуемое значение?

using System;
using System.Data;
using System.Data.SQLite;
using System.IO;
using System.Windows.Forms;

namespace WindowsFormsApplication1
{
    public partial class JournalEntries : Form
    {
        private SQLiteConnection sql_con;
        private SQLiteCommand sql_cmd;
        private DataSet DS = new DataSet();
        private DataTable DT = new DataTable();
        private string sPath = Path.Combine(Application.StartupPath, "mybd.db");

        public JournalEntries()
        {
            InitializeComponent();
        }

        private void JournalEntries_Load(object sender, EventArgs e)
        {
            string ConnectionString = @"Data Source=" + sPath + ";New=False;Version=3";
            selectTable(ConnectionString);
            // выбрать значения из справочников для отображения в comboBox
            String selectOS = "Select idOS, Name, Cost from OS";
            selectCombo(ConnectionString, selectOS, comboBox1, "Name", "idOS");
            String selectSubd = "SELECT idSubdivision, Name FROM Subdivision";
            selectCombo(ConnectionString, selectSubd, comboBox2, "Name", "idSubdivision");
            String selectMOL = "SELECT idMOL, Name FROM MOL";
            selectCombo(ConnectionString, selectMOL, comboBox3, "Name", "idMOL");
            comboBox1.SelectedIndex = -1;
            comboBox2.SelectedIndex = -1;
            comboBox3.SelectedIndex = -1;
        }

        private void button1_Click(object sender, EventArgs e)
        {
            string ConnectionString = @"Data Source=" + sPath + ";New=False;Version=3";
            // находим максимальное значение кода проводок для записи первичного ключа
            String mValue = "select MAX(IDJournalEntries) from JournalEntries";
            object maxValue = selectValue(ConnectionString, mValue);
            if (Convert.ToString(maxValue) == "")
                maxValue = 0;
            // Обнулить значения переменных
            string sum = "0";
            string count = "0";
            string coment = null;
            string Value1 = null;
            string Value2 = null;
            string Value3 = null;
            if (comboBox1.Text != "")
            {
                //ОС
                Value1 = comboBox1.SelectedValue.ToString();
            }
            if (comboBox2.Text != "")
            {
                //Подразделение
                Value2 = comboBox2.SelectedValue.ToString();
            }
            if (comboBox3.Text != "")
            {
                //МОЛ
                Value3 = comboBox3.SelectedValue.ToString();
            }
            //Поле количество
            if (textBox3.Text != "")
            {
                count = textBox3.Text;
            }
            //Поиск по базе данных значений
            String selectCost = "select Cost from OS where idOS='" + Value1 + "'";
            double Summa = Convert.ToDouble(selectCost) *
           Convert.ToDouble(count);
            String selectDT = "select idChart from ChartOfAccounts where Account = '''01'''";
 object DT = selectValue(ConnectionString, selectDT);
            String selectKT = "select idChart from ChartOfAccounts where Account = '''00'''";
 object KT = selectValue(ConnectionString, selectKT);
            string add = "INSERT INTO JournalEntries (IDJournalEntries, Date, DT, SubkontoDT1, SubkontoDT2, SubkontoDT3, KT, Count, Summa, IDOperation, Comment) VALUES(" + (Convert.ToInt32(maxValue) + 1) +
maskedTextBox1.Text + "'," + DT.ToString() + "," + Convert.ToInt32(Value1)
+ "," + Convert.ToInt32(Value2) + "," + Convert.ToInt32(Value3) + "," +
KT.ToString() + "," + Convert.ToDouble(count) + "," + Summa + ", " +
Convert.ToInt32(textBox1.Text) + ", " + ",'Поступление ОС','" + ")";
            ExecuteQuery(add);
            selectTable(ConnectionString);
        }
        // метод выполнения запроса к базе данных
        private void ExecuteQuery(string txtQuery)
        {
            sql_con = new SQLiteConnection("Data Source=" + sPath + ";Version=3;New=False;Compress=True;");
            sql_con.Open();
            sql_cmd = sql_con.CreateCommand();
            sql_cmd.CommandText = txtQuery;
            sql_cmd.ExecuteNonQuery();
            sql_con.Close();
        }
        public void selectTable(string ConnectionString)
        {
           
                SQLiteConnection connect = new
                SQLiteConnection(ConnectionString);
                connect.Open();
                SQLiteDataAdapter dataAdapter = new
                SQLiteDataAdapter("Select IDJournalEntries, Date, DT, SubkontoDT1, SubkontoDT2, SubkontoDT3, KT, SubkontoKT1, SubkontoKT2, SubkontoKT3, Count, Summa, IDOperation, Comment from JournalEntries", connect);
                DataSet ds = new DataSet();
                dataAdapter.Fill(ds);
                dataGridView1.DataSource = ds;
                dataGridView1.DataMember = ds.Tables[0].ToString();
                connect.Close();
                dataGridView1.Columns["IDJournalEntries"].DisplayIndex = 0;
                dataGridView1.Columns["Date"].DisplayIndex = 1;
                dataGridView1.Columns["DT"].DisplayIndex = 2;
                dataGridView1.Columns["SubkontoDT1"].DisplayIndex = 3;
                dataGridView1.Columns["SubkontoDT2"].DisplayIndex = 4;
                dataGridView1.Columns["SubkontoDT3"].DisplayIndex = 5;
                dataGridView1.Columns["KT"].DisplayIndex = 6;
                dataGridView1.Columns["SubkontoKT1"].DisplayIndex = 7;
                dataGridView1.Columns["SubkontoKT2"].DisplayIndex = 8;
                dataGridView1.Columns["SubkontoKT3"].DisplayIndex = 9;
                dataGridView1.Columns["Count"].DisplayIndex = 10;
                dataGridView1.Columns["Summa"].DisplayIndex = 11;
                dataGridView1.Columns["IDOperation"].DisplayIndex = 12;
                dataGridView1.Columns["Comment"].DisplayIndex = 13;
            
        }
        public void selectCombo(string ConnectionString, String selectCommand, ComboBox comboBox, string displayMember, string valueMember)
        {
            SQLiteConnection connect = new
            SQLiteConnection(ConnectionString);
            connect.Open();
            SQLiteDataAdapter dataAdapter = new
            SQLiteDataAdapter(selectCommand, connect);
            DataSet ds = new DataSet();
            dataAdapter.Fill(ds);
            comboBox.DataSource = ds.Tables[0];
            comboBox.DisplayMember = displayMember;
            comboBox.ValueMember = valueMember;
            connect.Close();
        }
        public object selectValue(string ConnectionString, String selectCommand)
        {
            SQLiteConnection connect = new
           SQLiteConnection(ConnectionString);
            connect.Open();
            SQLiteCommand command = new SQLiteCommand(selectCommand, connect);
            SQLiteDataReader reader = command.ExecuteReader();
            object value = "";
            while (reader.Read())
            {
                value = reader[0];
            }
            connect.Close();
            return value;
        }

        private void button2_Click(object sender, EventArgs e)
        {
            comboBox1.SelectedIndex = -1;
            comboBox2.SelectedIndex = -1;
            comboBox3.SelectedIndex = -1;
            textBox1.Clear();
            textBox2.Clear();
            textBox3.Clear();
            maskedTextBox1.Clear();
        }
    }
}