Monitor and alerting tool for specific queries I need to monitor and alert a MySQL DB which is hosted on AWS. I'd like an alert to trigger if certain SQL query criteria aren't met (e.g. SELECT * FROM my_table returns less than 100 results). 

What open-source tools are available for this type of monitoring? Most I've come across tend to monitor low level MySQL DB stats but I'd like specific queries to trigger alerts. I've come across Prometheus but was wondering if there others I could look into?
Nagios is an open source solution for Unix/Linux based systems.  It operates by scripts so it can do what you are requesting and more.  You can write your own script tailored to your specific alert (and you can set the warning and critical criteria). 

Here is a link to MySQL specific plugins freely available for your use:
https://exchange.nagios.org/directory/Plugins/Databases/MySQL

And a link to other similar opensource alerting products:
https://serverfault.com/questions/83963/alternatives-to-nagios