Don't know how to build task 'db:create'

Just figured it out. Someone had commented out the following in application.rb:

require 'rails/all'

Once put back rake -T returns the db tasks.

For people landing here in 2019:

If you ran rails new with -O (Capital 'O') or --skip-activerecord and want to add a database later, you need to uncomment this in application.rb to enable the Active Record framework:

require "active_record/railtie"

Edit:
You will also need to create the following structure in your application's root dir:

your_app
  |_config
    |_ database.yml
  |_db
    |_ migrate
    |_ schema.rb

then run:
rake db:schema:dump