代わりにこれを試してください:
private String readCommand =
"SELECT LEVEL FROM USERS WHERE VAL_1 = @param_val_1 AND VAL_2 = @param_val_2;";
public bool read(string id)
{
level = -1;
MySqlCommand m = new MySqlCommand(readCommand);
m.Parameters.AddWithValue("@param_val_1", val1);
m.Parameters.AddWithValue("@param_val_2", val2);
level = Convert.ToInt32(m.ExecuteScalar());
return true;
}