[sql/en] More database-agnostic SQL

[bowmanjd]

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Fixes #4602
A lot of changes on this one that I hope promote discussion:

• I tested my changes on MySQL, MariaDB, PostgreSQL, Microsoft SQL Server, SQLite, IBM DB2, and Oracle. If a statement doesn't work or works differently on one of these engines, I noted it.
• Instead of referencing the MySQL-specific employees database, I introduced CREATE and INSERT statements that populate a minimalist data sample for querying later, and altered all subsequent queries accordingly.
• I favored ANSI and then explain variants where appropriate, such as with ORDER BY... OFFSET... FETCH... vs LIMIT vs TOP
• I added a link to the Sakila database which is a fuller option and supports a wide range of engines

Questions:

• How best to show variants, such as LIMIT or TOP? Should they be commented or left as-is? They will not run on certain engines.
• Is the created-on-the-fly sample tables a good idea, or is it better to just use something like the Sakila sample database throughout
• Should we be describing the CREATE DATABASE and USE syntax at all, since it varies so much across engines?

[bowmanjd]

Would it be more helpful if I broke these changes out into multiple PRs? Happy to do so if anything here is helpful.

[verhovsky]

Sorry but this makes the article longer for no real benefit for most people. The link to http://troels.arvin.dk/db/rdbms should be enough for the people who need to know about it.