Syntax:
CONCAT(str1,str2,...)

Returns the string that results from concatenating the arguments. May
have one or more arguments. If all arguments are nonbinary strings, the
result is a nonbinary string. If the arguments include any binary
strings, the result is a binary string. A numeric argument is converted
to its equivalent nonbinary string form.

CONCAT() returns NULL if any argument is NULL.

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

