Getting this SQL Error: GROUP BY incompatible with sql_mode=only_full_group_by
MySQL 5.7.5+ changed the way GROUP BY behaved in order to be SQL99 compliant (where in previous versions it was not).

There is a good overview of the changes here.

Unfortunately, this affects some core queries that Craft uses when fetching elements.

The good news is that this has been fixed already in Craft 3 since we refactored the way that elements query grabs data and it no longer uses a GROUP BY.

The bad news is that it's going to be pretty impractical to backport that fix into Craft 2.

The workaround for Craft 2, if you're running MySQL 5.7.5+ is to edit your my.cnf file and remove the ONLY_FULL_GROUP_BY option from sql_mode. That will change GROUP BY behavior back to its pre-MySQL 5.7.5 behavior.

Update:

If, for whatever reason (primarily you're on shared hosting), you aren't able to change the sql_mode of your box at the environment level, as of the next Craft release (post 2.6.2945), there is a new initSQLs database config settings you can set in your craft/config/db.php.  It will accept an array of SQL statements to be executed immediately after the database connection has been established.  If you're running into this MySQL 5.7 error, you can do:

'initSQLs' =&gt; array("SET SESSION sql_mode='STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION';")


Just keep in mind this method will add at least one extra SQL query to every request.