Syntax:
AVG([DISTINCT] expr) [over_clause]

Returns the average value of expr. The DISTINCT option can be used to
return the average of the distinct values of expr.

If there are no matching rows, AVG() returns NULL.

This function executes as a window function if over_clause is present.
over_clause is as described in
https://dev.mysql.com/doc/refman/8.0/en/window-functions-usage.html; it
cannot be used with DISTINCT.

URL: https://dev.mysql.com/doc/refman/8.0/en/aggregate-functions.html

