The case statement would only be there to set it to a numeric 0 when it's an unusual value like an apostrophe - which is what the OP said it did in a comment somewhere else in this discussion (https://news.ycombinator.com/item?id=14499589).
>> I have similar feelings about subqueries. This is a rather elementary standard join query, and I always felt subqueries were only tacked onto SQL because people had trouble understanding joins.
Not exactly sure what you meant by similar feelings (did you mean you also don't get the idea about it?). In the absence of knowing MySQL's execution plan for the OP's query, I tend to think using subqueries that reduce the rows of activity_types and activities would make the join, aggregation and ordering faster (assuming the tables are already properly indexed). I've seen this reduce query times on other database engines for similar types of queries.
I have similar feelings about subqueries. This is a rather elementary standard join query, and I always felt subqueries were only tacked onto SQL because people had trouble understanding joins.