Selecting multiple items from a checkbox and adding to an existing SQL Server query into an if statement in ASP.NET

I wanted to know if I can add to the existing SQL Server query in ASP.NET code. I have multiple drop-down menus, check boxes and buttons in the designer. I have a checkbox where if one or more items are selected, it adds to the query.

For example: I have a checkbox called PositionList with multiple positions: Sales, Marketing, Tech Support, HR, and Finance and I wish to create a table where I can select one or more item and get a result of people from those positions.

public partial class _Default : Page
{
    string connection1 = "Server = myserver" 
    string querystring1 = "select name, age, gender, race, city,convert(varchar, datehired, 100) as 'Date Hired', position from NewEmployeesList"
}
...

protected void btnnormal_Click(object sender, EventArgs e)
{
    SqlConnection conn1= new SqlConnection(connection1)
    SqlCommand command1 = new SqlCommand(queryString1, conn1);

    DataTable table1 = new DataTable();

    conn1.Open();
    command1.CommandTimeout = 300; 

    Data1.Fill(table1)

    string DoH = query
    GridView1.DataSource = table1;
    GridView1.DataBind();

    // the table works fine up until this point
    foreach (ListItem li in PositionList.Items)
    {
        if (li.Selected == true)
        { 
            queryString1 + " and position = " + PositionList.SelectedItem;
        }
    }

    // this is the part where I am a stuck, not sure how to add to the 
    // SQL Query properly and how I can check multiple checkboxes so it 
    // returns the table with those positions