You can buy the eBook from online retailers like:

While "Understanding SQL" is primarily a book rather than a single academic paper, Martin Gruber's work is frequently cited in educational curricula and technical documentation regarding relational databases. Understanding SQL (Book)

Use a tool like DB Fiddle or a local installation of PostgreSQL to run the examples.

If you are looking for specific "papers" or the updated PDF content, you can check the following authorized repositories: