Arduino

const int ledPin9 = 9;
const int ledPin8 = 8;
const int ledPin7 = 7;
const int ledPin6 = 6;
int pin;
int state;
 
void setup() {
  pinMode(ledPin9, OUTPUT);
  pinMode(ledPin8, OUTPUT);
  pinMode(ledPin7, OUTPUT);
  pinMode(ledPin6, OUTPUT);
  Serial.begin(9600);
 
}
 
void loop() {
  if (Serial.available() == 2) {
    Serial.write('a');
    pin = Serial.read();
    Serial.write(pin);
    state = Serial.read();
    Serial.write(state);
   
    if (pin == '1') {
      Serial.write('q');
      if (state == 'h') {
        Serial.write('h');
        digitalWrite(ledPin9, HIGH);
      } else {
        Serial.write('l');
        digitalWrite(ledPin9, LOW);
      }
    }
    if (pin == '2') {
      Serial.write('q');
      if (state == 'h') {
        Serial.write('h');
        digitalWrite(ledPin8, HIGH);
      } else {
        Serial.write('l');
        digitalWrite(ledPin8, LOW);
      }
    }
    if (pin == '3') {
      Serial.write('q');
      if (state == 'h') {
        Serial.write('h');
        digitalWrite(ledPin7, HIGH);
      } else {
        Serial.write('l');
        digitalWrite(ledPin7, LOW);
      }
    }
    if (pin == '4') {
      Serial.write('q');
      if (state == 'h') {
        Serial.write('h');
        digitalWrite(ledPin6, HIGH);
      } else {
        Serial.write('l');
        digitalWrite(ledPin6, LOW);
      }
    }
  }
  delay(500);
}

Raspberry.py

import mysql.connector
import serial
import time

lastIdState=[2,2,2,2]

try:
        ser = serial.Serial('/dev/ttyUSB0', 9600)
        print ('Arduino connection established')
except Exception,e:
        print str(e)
print ('Please wait...')
time.sleep(5)
print ('Configuration OK. Starting While')

while True:
        print ('Iteration begin')
        try:
                con = mysql.connector.connect(user='root', password='***', host='127.0.0.1', database='aquarium')
                print ('MySQL connection established')
                cursor = con.cursor()
                cursor.execute("SELECT * FROM relay")
                results = cursor.fetchall()
                print (results)
                i = 0
                for row in results:
                        if(lastIdState[i] != row[1]):
                                lastIdState[i] = row[1]
                                ser.write(str(row[0]))
                                if(row[1] == 1):
                                        ser.write('h')
                                        print('Turning On: ' + str(row[0]))
                                elif(row[1] == 0):
                                        ser.write('l')
                                        print('Turning Off: ' + str(row[0]))
                                else:
                                        print ('State Error: ' + str(row[1]))
                        time.sleep(0.5)
                        i += 1
        except Exception,e:
                print str(e)
        finally:
                con.close()
                print ('MySQL connection closed')
        print ('Iteration end\n')
        time.sleep(1)