I think it's safe to say that when we're discussing databases in the context of industrial process control, SQL Server is where most of our brains tend to jump. The proliferation of SQL (and other traditional relational databases) across most, if not all, industries for process control applications cannot be denied. But SQL (and others) do certainly have some limitations.
This blog post (based on an article by partner company, inray Industriesoftware GmbH) discusses how MongoDB, a non-relational database (i.e. NoSQL database) are fundamentally different from the SQL Servers of the world and how MongoDB can be a good alternative to relational databases for some applications.