This example provides sample configuration for creating a mysql or postgres insatnce. For Oracle/SQL Servers, replace default values with appropriate values, they are not included in sample since the number of options are high.
The example creates db subnet groups and a VPC security group as inputs to the instance creation
For AWS provider, set up your AWS environment as outlined in https://www.terraform.io/docs/providers/aws/index.html
If you need to use existing security groups and subnets, remove the sg.tf and subnets.tf files and replace the corresponidng sections in main.tf under aws_db_instance
Several paraneters are externalized, review the different variables.tf files and change them to fit your needs. Carefully review the CIDR blocks, egress/ingress rules, availability zones that are very specific to your account.
Once ready run 'terraform plan' to review. At the minimum, provide the vpc_id as input variable.