you haven't closed yo if statement mate if(Rd1.read()).
This should actually be a while statement while rd1 is reading do this when there is no more data to read stop:
rd1 = cmd1.ExecuteReader();
while (rd1.Read())
{
string z;
z = rd1.GetValue(0).ToString();
for (int i = 0; i <= (checkedListBox1.Items.Count - 1); i++)
{
checkedListBox1.SetItemChecked(i, false);
string x = checkedListBox1.Items[i].ToString();
if (z == x)
{
checkedListBox1.SetItemChecked(i, true);
}
}
try something like that