It's typical for a column called "id" (or "Id") to be the primary key field of the given table, so grouping by Id would be even more silly than using an "order by" clause on a query that returns just one row.

I thought i addressed that with Because of that, the ORDER BY clause makes no sense. Perhaps i need to be clear next time.

"id" (or "Id")

Databases capitalize all non-quoted object names. (SQL-generators use quotes to be safe.) So, it's just a matter of personal preference. As an aside, typical SQL developers' convention is the refer to clauses without the quotes, with some capitalizing them for clarity.

an "order by" clause can refer to any column in the table being queried, regardless whether that column is used in any way elsewhere in the query.

I don't know about mysql, but a query with an aggregate function cannot refer to any column--even in the ORDER BY-- unless it is specified in the GROUP BY. Logically, it makes no sense. That is, unless you are grouping by it, the column doesn't exist in the data set!

having it in there is a harmless mistake in this case.

I don't know mysql, but on every other RDBMS it would be generating an error.