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.
So, steps to success:
sudo vim /etc/mysql/my.cnf
sql_mode=STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTIONto the bottom of the file
sudo service mysql restartto restart MySQL.