Today, just about every useful database system supports SQL to some extent. In theory, SQL acts as a good unifier, since database applications written to use SQL as the interface to the database can be ported to other database systems with little cost in terms of time and effort. Commercial pressures however, dictate that database manufacturers distinguish their products one from another. This has led to SQL variations, not helped by the fact that the standard for SQL does not define commands for many of the database administration tasks that are an essential part of using a database in the real world. So, there are differences between the SQL used by Oracle, SQL Server, PostgreSQL, and other database systems.
I have a little bit of a hard time understanding what exactly that's trying to say. I presume that this is an adverbial type of clause which explains why the situation mentioned above has led to different versions of the SQL language being available on the market instead of just one, but I'm not one hundred percent sure about that though. Plus, I don't think I understand what the actual words actually mean.