Syntax:
CONCAT_WS(separator,str1,str2,...)

CONCAT_WS() stands for Concatenate With Separator and is a special form
of CONCAT(). The first argument is the separator for the rest of the
arguments. The separator is added between the strings to be
concatenated. The separator can be a string, as can the rest of the
arguments. If the separator is NULL, the result is NULL.

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

