>I'm no SQL guru, but won't that subselect will break down when users and events grow to the millions? A moderately decent database server would have no issues

>I'm no SQL guru, but won't that subselect will break down when users and events grow to the millions? A moderately decent database server would have no issues in such a case, however yes, optimization would be case-specific, just as it would be with MongoDB. I was simply comparing readability of SQL for the example that you gave.