RDS subnet group vs VPC

main.tf

terraform {
  required_version = ">= 0.11"
}

provider "aws" {
  region  = "${var.aws_region}"
  version = "~> 1.6"
}

variable "aws_region" {
  default = "ap-southeast-1"
}

variable "aws_azs" {
  default = [
    "ap-southeast-1a",
    "ap-southeast-1b",
  ]
}

resource "aws_vpc" "vpc_one" {
  cidr_block = "10.0.0.0/16"

  tags = {
    Name = "VPC 1"
  }
}

resource "aws_vpc" "vpc_two" {
  cidr_block = "10.50.0.0/16"

  tags = {
    Name = "VPC 2"
  }
}

resource "aws_subnet" "vpc_one" {
  count = "${length(var.aws_azs)}"

  vpc_id     = "${aws_vpc.vpc_one.id}"
  cidr_block = "10.0.${count.index}.0/24"

  availability_zone = "${element(var.aws_azs, count.index)}"

  tags = {
    Name = "Subnet 1-${count.index}"
  }
}

resource "aws_subnet" "vpc_two" {
  count = "${length(var.aws_azs)}"

  vpc_id     = "${aws_vpc.vpc_two.id}"
  cidr_block = "10.50.${count.index}.0/24"

  availability_zone = "${element(var.aws_azs, count.index)}"

  tags = {
    Name = "Subnet 2-${count.index}"
  }
}

resource "aws_db_subnet_group" "vpc_one" {
  name       = "sg_one"
  subnet_ids = ["${aws_subnet.vpc_one.*.id}"]

  tags = {
    Name = "SG 1"
  }
}

resource "aws_db_subnet_group" "vpc_two" {
  name       = "sg_two"
  subnet_ids = ["${aws_subnet.vpc_two.*.id}"]

  tags = {
    Name = "SG 2"
  }
}

resource "aws_db_parameter_group" "default" {
  name   = "rds-mysql"
  family = "mysql5.6"
}

# the db_subnet_group_name can be freely changed between the existing subnets
resource "aws_db_instance" "default" {
  allocated_storage    = 10
  storage_type         = "gp2"
  engine               = "mysql"
  engine_version       = "5.6.37"
  instance_class       = "db.t2.micro"
  name                 = "mydb"
  username             = "root"
  password             = "r00tr00t"
  db_subnet_group_name = "${aws_db_subnet_group.vpc_one.name}"
  parameter_group_name = "${aws_db_parameter_group.default.name}"
}