Can't store the values I get from my temperature in the database.

Using Arduino Sensors
6

you don't check the return values of any call (aka good weather programming)

sp1 = new SerialPort("COM4", 9600);
rv = sp1.open();

why do you assume sp1 = new SerialPort("COM4", 9600); returns a valid value?

sp1 = new SerialPort("COM4", 9600);

if (sp1 != null) 
{  
  try
  {
    rv = sp1.open();
  }
  catch()
  {
    // open failed ...
  }
}