Explain this statement about filtering in SQL Server vs MySQL
"Very few syntax changes" -- Excuse my while I laugh!  SQL Server has a lot of features that MySQL does not have.  Any use of such is not just a syntax change, but possibly a query rewrite.

TOP 10 --> LIMIT 10 -- but only approximately.

ROW_NUMBER --> does not exist

SEQUENCE --> AUTO_INCREMENT -- but only approximately

dbname.dbo.tablename --> dbname.tablename

Which version of MySQL?  MySQL 8.0 has a good chance of having some of the fancier features being used in SQL Server.  MariaDB 10.4 is likely to come even closer to SQL Server, in that it handles some of the incompatibilities that MySQL does not cover.

JOINing across databases on the same instance of MySQL has never been a problem:

SELECT a.this, b.that
    FROM db1.tblx AS a
    JOIN db2.tbly AS b
    WHERE a.foo = 1
      AND b.blat &lt; 5;


JOINing across two separate instances (of MySQL or MariaDB) on the same server, or separate instances on separate servers is a problem.  See MariaDB's "Connect".  The quote you provided leaves the term "database" ambiguous.  (Or it was pulled out of context.)  I hope that I have made it clear with the addition of the term "instance".

As for the rest of that link.  It is aimed at CEOs, not us grunts who will have to make the transition.  Too much 'hand-waving' and fluff; no meat.

I have not figured out what it means by "row-based filtering".  Maybe it is a snazzy feature in SQL Server.  But maybe it is not being used by your code.

Start a new Question and post in it "how do I change this query to MySQL" together with a non-trivial, but typical query.  You may see some of the 4 incompatibilities I mentioned above, or there may be other things waiting to trip up the conversion.