Syntax:
SOUNDEX(str)

Returns a soundex string from str, or NULL if str is NULL. Two strings
that sound almost the same should have identical soundex strings. A
standard soundex string is four characters long, but the SOUNDEX()
function returns an arbitrarily long string. You can use SUBSTRING() on
the result to get a standard soundex string. All nonalphabetic
characters in str are ignored. All international alphabetic characters
outside the A-Z range are treated as vowels.

*Important*:

When using SOUNDEX(), you should be aware of the following limitations:

o This function, as currently implemented, is intended to work well
  with strings that are in the English language only. Strings in other
  languages may not produce reliable results.

o This function is not guaranteed to provide consistent results with
  strings that use multibyte character sets, including utf-8. See Bug
  #22638 for more information.

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

