Access mysql in other container

docker-compose.yml

mysql:
  image: mysql:latest
  container_name: mysql
  environment:
    - MYSQL_ROOT_PASSWORD=root_password
    - MYSQL_USER=user
    - MYSQL_PASSWORD=user_pass
    - MYSQL_DATABASE=user_db
app:
  image: mysql:latest
  container_name: example_app
  links:
    - mysql:db
  command: bash

Running example:

$ docker-compose run app
Creating mysql
root@cd9cf6553130:/# mysql -h "$DB_PORT_3306_TCP_ADDR" -P "$DB_PORT_3306_TCP_PORT" -u "$DB_ENV_MYSQL_USER" --password="$DB_ENV_MYSQL_PASSWORD" $DB_ENV_MYSQL_DATABASE
mysql: [Warning] Using a password on the command line interface can be insecure.
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 2
Server version: 5.7.10 MySQL Community Server (GPL)

Copyright (c) 2000, 2015, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> exit
Bye
root@cd9cf6553130:/#