Try to use only standard SQL?

Stopped reading right there.

There is no way you can port SQL Server to MySQL to Oracle to PLSQL without rewriting virtually every single query. They're all too different.

So why bother? Write idiomatic code that other programmers used to that engine will understand.

Not only that but different engines like different things. That blazingly fast nested subquery in MS SQL will become a massive performance problem in MySQL.

There is no such thing as portable SQL, don't pretend there is.

Not only that, often the standardised way is worse than the vendor specific way (e.g. INFORMATION_SCHEMAS vs built in sp_ from MS).