The result of GROUP_CONCAT is truncated to the maximum length that is given by the group_concat_max_len system variable, which has a default value of 1024.

This will cause  data-destroying bugs in production. For this reason you should probably not use GROUP_CONCAT . At least you must set the value of group_concat_max_len to an insanely high value on every database server your application runs on.

Like MAX or COUNT, GROUP_CONCAT is a MySQL aggregation function you can use whenever your query contains a GROUP BY. You can use it to retrieve a comma-separated list of all values in a given column within each group.

select  client
,Month
,year
,Sum(Amount) as Amount_Total
,GROUP_CONCAT(Amount) as Amount_Detail
,GROUP_CONCAT(InvoiceNum) as Invoices
from mytable
 group by Client

The general syntax of Creating a Function is :

 CREATE FUNCTION func_name ([func_parameter[,...]]) RETURNS type routine_body

func_name : Function name
func_parameter : param_name type
type : Any valid MySQL datatype
routine_body : Valid SQL procedure statement

 The RETURN clause is mandatory for FUNCTION . It used to indicate the return type of function.

Now we are describing you a simple example a function. This function take a parameter and it is used to perform an operation by using an SQL function and return the result. In this example there is no need to use delimiter because it contains no internal ; statement delimiters. Example :

CREATE FUNCTION func(str CHAR(20))
RETURNS CHAR(50)
RETURN CONCAT('WELCOME TO, ',str,'!');

Query OK, 0 rows affected (0.00 sec)

 SELECT func('Galhano.com');

+------------------------+
| func('Galhano.com')      |
+------------------------+
| WELCOME TO, Galhano.com |
+------------------------+
1 row in set (0.00 sec)

http://dev.mysql.com/doc/refman/5.0/en/create-procedure.html

http://www.roseindia.net/mysql/mysql5/stored-procedures-and-functions.shtml