Turning on aggregration causes a SQL error
Aggregation can be sort of a pain in Drupal 8's Views implementation. Part of this is due to limitations in how MySQL handles GROUP BY.

As @Jonathan points out, for an aggregation function to work at all, you need to have at least one group by expression.  And if you want a function like COUNT or MIN to work as you expect, you only want the one group to be listed in the GROUP BY.

To get this to work right just now, you need to:


Under Advanced, you need to set Aggregation to "yes".
Under Format > Settings, you need to choose your Group by field.
You may not sort by anything other than the field you chose under Format > Settings, or your aggregation functions will give you bogus answers.


This is a bit limiting for some things you'll want to do, but as of 8.0.2, it's tricky to get better than this.

If anybody knows how to do better than this, I'd love to hear how :-)