Change JOIN tutorial to focus more on explicit joins

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From: PG Doc comments form <noreply@postgresql.org>
To: pgsql-docs@lists.postgresql.org
Cc: shammat@gmx.net
Date: 2020-05-20T10:07:03Z
Lists: pgsql-hackers, pgsql-docs
The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/12/tutorial-join.html
Description:

The tutorial about joins makes the following statement about the explicit
JOIN operator:

> This syntax is not as commonly used as the one above

I think in 2020 this claim is no longer true, and I would love to see the
manual prefer the "modern" explicit JOIN operator rather than sticking to
the ancient implicit joins in the WHERE clause.

Commits

  1. doc: Prefer explicit JOIN syntax over old implicit syntax in tutorial

  2. doc: Change table alias names to lower case in tutorial chapter

  3. doc: Fix whitespace issue in PDF

  4. doc: Use tags consistently in the tutorial chapter