free:
- SQL on Khan Academy: https://www.khanacademy.org/computing/computer-programming/sql
- Codeacademy Learn SQL: https://www.codecademy.com/en/courses/learn-sql
- SQL Teaching: http://www.sqlteaching.com
- SQL Bolt: http://sqlbolt.com
- PostgreSQL Exercises: http://pgexercises.com
- SQL Zoo: http://sqlzoo.net/wiki/SQL_Tutorial
- PostgreSQL Documentation: http://www.postgresql.org/docs/9.4/interactive/index.html
You could add curated sql resources from Hackr.io to your Wiki - https://hackr.io/tutorials/learn-sql