Start MySQL client as root user:
$ sudo mysql -u root
See existing users:
> SELECT User, Host from mysql.user;
Create new database and user:
> CREATE DATABASE mydb;
> CREATE USER 'myuser'@'localhost' IDENTIFIED BY 'mypass';
> GRANT ALL PRIVILEGES ON `mydb`.* TO 'myuser'@'localhost';
Start PostgreSQL client as postgres user:
$ sudo -u postgres psql
Create user and database:
# CREATE DATABASE mydb;
# CREATE USER myuser WITH ENCRYPTED PASSWORD 'mypass';
# GRANT ALL PRIVILEGES ON DATABASE mydb TO myuser;