Syntax:
COUNT(expr) [over_clause]

Returns a count of the number of non-NULL values of expr in the rows
retrieved by a SELECT statement. The result is a BIGINT value.

If there are no matching rows, COUNT() returns 0. COUNT(NULL) returns
0.

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.

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

