To get the max/min/avg-value from a selected interval in Grafana when using MySQL as data storage just add the following expression to your SQL query:
<SQL-QUERY> WHERE $__timeFilter(<time-column>)
Replace <time-column> with the name of your column where the time data is stored.
After that navigate to Value options>Calculation and select MAX/MIN/AVG.
Sourced from grafana.com - table queries and community.grafana.com - min-max-values