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

Returns the string that results from concatenating the arguments. May
have one or more arguments. If all arguments are non-binary strings,
the result is a non-binary string. If the arguments include any binary
strings, the result is a binary string. A numeric argument is converted
to its equivalent binary string form; if you want to avoid that, 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/5.1/en/string-functions.html

