How to handle the error when using DataGridView controls and Access database queries?

Question

I have two controls DataGridView.

For the second, I just copied the code from the first and changed where the difference is. But in the second, an error appears when I want to view the result of my SQL code.
The error given in English shows something like that there was no value given to at least one required parameter.

private void button1_Click(object sender, EventArgs e)
{
    string connectionString = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=save.mdb";

    try
    {
        database = new OleDbConnection(connectionString);
        database.Open();
        date =  DateTime.Now.ToShortDateString();

        string queryString = "SELECT zivila.naziv,(obroki_save.skupaj_kalorij/zivila.kalorij)*100 as Kolicina_v_gramih "
               + "FROM (users LEFT JOIN obroki_save ON obroki_save.ID_uporabnika=users.ID) "
               + "LEFT JOIN zivila ON zivila.ID=obroki_save.ID_zivila"
               + "WHERE users.ID= " + a.ToString();

        loadDataGrid(queryString);
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.Message);
        return;
    }
}

public void loadDataGrid(string sqlQueryString)
{
    OleDbCommand SQLQuery = new OleDbCommand();
    DataTable data = null;
    dataGridView1.DataSource = null;
    SQLQuery.Connection = null;
    OleDbDataAdapter dataAdapter = null;
    dataGridView1.Columns.Clear(); // <-- clear columns

    SQLQuery.CommandText = sqlQueryString;
    SQLQuery.Connection = database;
    data = new DataTable();
    dataAdapter = new OleDbDataAdapter(SQLQuery);
    dataAdapter.Fill(data);
    dataGridView1.DataSource = data;
    dataGridView1.AllowUserToAddRows = false;
    dataGridView1.ReadOnly = true;
    dataGridView1.Columns[0].Visible = true;
}

private void button2_Click(object sender, EventArgs e)
{
    string connectionString = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=save.mdb";

    try
    {
        database = new OleDbConnection(connectionString);
        database.Open();
        date = DateTime.Now.ToShortDateString();

        string queryString = "SELECT skupaj_kalorij  "
                + "FROM obroki_save "
                + "WHERE users.ID= " + a.ToString();

        loadDataGrid2(queryString);
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.Message);
        return;
    }
}

public void loadDataGrid2(string sqlQueryString)
{
    OleDbCommand SQLQuery = new OleDbCommand();
    DataTable data = null;
    dataGridView2.DataSource = null;
    SQLQuery.Connection = null;
    OleDbDataAdapter dataAdapter = null;
    dataGridView2.Columns.Clear(); // <-- clear columns

    SQLQuery.CommandText = sqlQueryString;
    SQLQuery.Connection = database;
    data = new DataTable();
    dataAdapter = new OleDbDataAdapter(SQLQuery);
    dataAdapter.Fill(data);
    dataGridView2.DataSource = data;
    dataGridView2.AllowUserToAddRows = false;
    dataGridView2.ReadOnly = true;
    dataGridView2.Columns[0].Visible = true;
}

Answer 1

: http://msdn.microsoft.com/en-us/library/ms171884(v=VS.80).aspx , 20

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Answer 2

, private void button2_Click()

a.ToString();

?

Answer 3

a.Tostring():

string queryString = "SELECT zivila.naziv,(obroki_save.skupaj_kalorij/zivila.kalorij)*100 as Kolicina_v_gramih "
               + "FROM (users LEFT JOIN obroki_save ON obroki_save.ID_uporabnika=users.ID)"
               + " LEFT JOIN zivila ON zivila.ID=obroki_save.ID_zivila "
               + " WHERE users.ID= '" + a.ToString() + "'";

string.format():

string queryString = string.format("SELECT zivila.naziv,(obroki_save.skupaj_kalorij/zivila.kalorij)*100 as Kolicina_v_gramih "
               + "FROM (users LEFT JOIN obroki_save ON obroki_save.ID_uporabnika=users.ID)"
               + " LEFT JOIN zivila ON zivila.ID=obroki_save.ID_zivila "
               + " WHERE users.ID= '{0}'", a);