From MySQL's docs, versions less than 5.7.5 support SQL-92's version of
GROUP BY, which requires all non-aggregate columns to show up in the
GROUP BY.

> SQL:1999 and later permits such nonaggregates per optional feature T301
> if they are functionally dependent on GROUP BY columns: If such
> a relationship exists between name and custid, the query is legal. This
> would be the case, for example, were custid a primary key of customers.
> MySQL 5.7.5 and later implements detection of functional dependence.
> https://dev.mysql.com/doc/refman/5.7/en/group-by-handling.html

Turns out Travis hasn't been testing on 5.6, which explains why it was
working (to my bewilderment, as I know this is an issue).

Thanks @augensalat for reporting this issue!

Fixes #36

• Commit 5625b71
• Compare 2f692fb..5625b71
• Branch master

Doug Bell