Syntax:
FROM_BASE64(str)

Takes a string encoded with the base-64 encoded rules used by
TO_BASE64() and returns the decoded result as a binary string. The
result is NULL if the argument is NULL or not a valid base-64 string.
See the description of TO_BASE64() for details about the encoding and
decoding rules.

URL: https://dev.mysql.com/doc/refman/5.7/en/string-functions.html

