在AWS EC2实例中创建并连接到公共子网上的AWS RDS数据库的代码示例如下：

1. 创建RDS数据库实例：

import boto3

client = boto3.client('rds')

response = client.create_db_instance(
    DBInstanceIdentifier='my-rds-instance',
    AllocatedStorage=20,
    DBInstanceClass='db.t2.micro',
    Engine='mysql',
    MasterUsername='',
    MasterUserPassword='',
    VpcSecurityGroupIds=[''],
    AvailabilityZone='us-west-2a',
    PubliclyAccessible=True,
    MultiAZ=False,
    StorageType='gp2',
    StorageEncrypted=False,
    Tags=[
        {
            'Key': 'Name',
            'Value': 'MyRDSInstance'
        },
    ],
)

2. 创建EC2实例并连接到RDS数据库：

import boto3

ec2_client = boto3.client('ec2')

response = ec2_client.run_instances(
    ImageId='',
    InstanceType='t2.micro',
    KeyName='',
    SecurityGroupIds=[''],
    SubnetId='',
    MinCount=1,
    MaxCount=1,
)

instance_id = response['Instances'][0]['InstanceId']

ec2 = boto3.resource('ec2')

instance = ec2.Instance(instance_id)

instance.wait_until_running()

print("EC2 instance is running.")

# 连接到RDS数据库
import pymysql

endpoint = ''
port = 3306
username = ''
password = ''
database = ''

conn = pymysql.connect(host=endpoint, port=port, user=username, passwd=password, db=database)

cursor = conn.cursor()

# 执行SQL查询
cursor.execute("SELECT * FROM my_table")

# 获取结果
result = cursor.fetchall()

# 打印结果
for row in result:
    print(row)

# 关闭数据库连接
conn.close()

请根据您的环境和需求修改代码中的参数和数值。确保在运行代码之前已正确配置AWS CLI，并具有适当的IAM权限。