how to select checkbox

If you want to insert the selected data in single column, you can use following code for storing the data into database. It will be inserting values with comma as a separator.

string selectedItems = string.Empty; 
foreach (ListItem item in this.CheckBoxList1.Items) 
{
    if (item.Selected) 
                selectedItems += item + ","; 
}
SqlCommand cmd = new SqlCommand("Insert into YOUR_TABLE(YOUR_COLUMN) values('" + selectedItems + "')", cn); 
cn.Open(); // I am assuming that you have written down the connection string. 
int iCount = cmd.ExecuteNonQuery(); 
cn.Close();

For selecting:

void LoadCheckList() 
    { 
        string selectedItems = ""; 
        SqlCommand cmd = new SqlCommand("Select YOUR_COLUMN from YOUR_TABLE", cn); // Write your Query i have taken as sample 
        SqlDataReader dr; 
        cn.Open(); 
        dr = cmd.ExecuteReader(); 
        if (dr.Read()) 
        { 
            selectedItems = dr["selectedItems"].ToString(); 
        }
        dr.Close();
        cn.Close(); 
        string[] arr = selectedItems.Split(','); 
        foreach (ListItem item in this.CheckBoxList1.Items) 
        { 
            foreach (string s in arr) 
            { 
                if (item.Text== s) 
                { 
                    item.Selected = true; 
                } 
            } 
        } 
    }