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 string form. This is a nonbinary string as of MySQL
5.5.3. Before 5.5.3, it is a binary string; to avoid that and produce a
nonbinary string, you can use an explicit type cast, as in this
example:

SELECT CONCAT(CAST(int_col AS CHAR), char_col);

CONCAT() returns NULL if any argument is NULL.

URL: http://dev.mysql.com/doc/refman/5.5/en/string-functions.html

