It happens to me using PHP 7.3 and WordPress 4.9.10.
It is a core issue. That said. I fixed it locally by adding the following code just belown line 820 of wp-includes/class-wp-comment-query.php
$groupby = ”;
It may not seem like a big deal, but it’s a very visible issue when you use comments on your site. This error notice also appears in the dashboard.
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This reply was modified 6 years, 9 months ago by Andrew Woods. Reason: corrected my wordpress version for accuracy
The plugin has been updated, including the readme. Go check it out.
