EC2内でMySQLに接続ができない

##困っっていること・前提
EC2に接続しMySQLに接続したいのですが接続ができません。

##エラー

$mysql -u admin -p -h ☓☓☓☓☓☓☓☓☓☓☓☓☓☓☓☓☓☓☓☓☓☓☓☓☓☓☓☓☓☓☓☓☓☓☓☓
>ERROR 2003 (HY000): Can't connect to MySQL server on '☓☓☓' (110)

##コード
sg.tf

resource "aws_security_group" "sample_sg" {
  vpc_id      = aws_vpc.sample_vpc.id
  name        = "sample_sg"
  description = "Define of SG for public"
  egress {
    from_port   = 0
    to_port     = 0
    protocol    = "-1"
    cidr_blocks = ["0.0.0.0/0"]
  }
  tags = {
    Name = "sample_sg"
  }
}

#セキュリティグループルール
resource "aws_security_group_rule" "inbound_http" {
  type        = "ingress"
  from_port   = 80
  to_port     = 80
  protocol    = "tcp"
  cidr_blocks = ["0.0.0.0/0"]
  # source_security_group_id = aws_security_group.sample_sg.id

  security_group_id = aws_security_group.sample_sg.id
}

resource "aws_security_group_rule" "inbound_ssh" {
  type        = "ingress"
  from_port   = 22
  to_port     = 22
  protocol    = "tcp"
  cidr_blocks = ["0.0.0.0/0"]

  security_group_id = aws_security_group.sample_sg.id
}

rds.tf

resource "aws_db_instance" "db" {
  identifier        = "sample-db"
  allocated_storage = 20
  storage_type      = "gp2"
  engine            = "mysql"
  engine_version    = "8.0.21"
  instance_class    = "db.t2.micro"
  multi_az          = true

  name     = "testdb"
  username = "admin"
  password = "admintest"

  final_snapshot_identifier = "aws-sample-db"

  db_subnet_group_name   = aws_db_subnet_group.sample_subnet_group_db.name
  vpc_security_group_ids = [aws_security_group.sample_sg.id]
  parameter_group_name   = aws_db_parameter_group.default.name
}

resource "aws_db_subnet_group" "sample_subnet_group_db" {
  name        = "sample_db"
  subnet_ids  = [aws_subnet.sample_private_1a.id, aws_subnet.sample_private_1c.id]
  description = "It is a DB subnet group on sample_vpc."
  tags = {
    Name = "sample_sg_db"
  }
}

#セキュリティグループの設定
resource "aws_security_group" "sample_sg_db" {
  vpc_id      = aws_vpc.sample_vpc.id
  name        = "db"
  description = "Allow DB inbound traffic"

  ingress {
    from_port   = 3306
    to_port     = 3306
    protocol    = "tcp"
    cidr_blocks = ["0.0.0.0/0"]
    # security_groups = [aws_security_group.sample_sg.id]
  }

  egress {
    from_port   = 0
    to_port     = 0
    protocol    = "-1"
    cidr_blocks = ["0.0.0.0/0"]
  }

  tags = {
    Name = "sample_allow_db"
  }
}

#データーベースのコンフィグの管理
resource "aws_db_parameter_group" "default" {
  name   = "rds-pg"
  family = "mysql8.0"
}

output "db_endpoint" {
  value = aws_db_instance.db.endpoint
}

output "db_name" {
  value = aws_db_instance.db.name
}

output "db_user" {
  value = aws_db_instance.db.username
}

output "db_password" {
  value = aws_db_instance.db.password
}

vpc.tf

resource "aws_vpc" "sample_vpc" {
  cidr_block           = "10.0.0.0/16"
  enable_dns_hostnames = true
  enable_dns_support   = true

  tags = {
    # type = string
    Name = "sample_vpc"
  }
}

#パブリックサブネット
resource "aws_subnet" "sample_public_1a" {
  vpc_id                  = aws_vpc.sample_vpc.id
  cidr_block              = "10.0.0.0/24"
  availability_zone       = "ap-northeast-1a"
  map_public_ip_on_launch = true

  tags = {
    # type = string
    Name = "public_subnet_1a"
  }

  timeouts {
    delete = "5m"
  }
}

#プライベートサブネット
resource "aws_subnet" "sample_private_1a" {
  vpc_id            = aws_vpc.sample_vpc.id
  cidr_block        = "10.0.1.0/24"
  availability_zone = "ap-northeast-1a"
  # map_public_ip_on_launch = true

  tags = {
    Name = "private_subnet_1a"
  }
}

resource "aws_subnet" "sample_private_1c" {
  vpc_id            = aws_vpc.sample_vpc.id
  cidr_block        = "10.0.2.0/24"
  availability_zone = "ap-northeast-1c"

  tags = {
    Name = "private_subnet_1c"
  }
}

#インターネットゲートウェイ
resource "aws_internet_gateway" "sample_igw" {
  vpc_id = aws_vpc.sample_vpc.id

  tags = {
    Name = "sample_igw"
  }
}

#ルートテーブルの定義
resource "aws_route_table" "sample_public" {
  vpc_id = aws_vpc.sample_vpc.id
  route {
    cidr_block = "0.0.0.0/0"
    gateway_id = aws_internet_gateway.sample_igw.id
  }
  tags = {
    Name = "sample_route_table"
  }
}

#ルートテーブルのサブネットの紐付け
resource "aws_route_table_association" "public_1a" {
  subnet_id      = aws_subnet.sample_public_1a.id
  route_table_id = aws_route_table.sample_public.id
}

db用のセキュリティグループ