mysql列转行(列值不确定

有这样的一需求：
b列的值是不确定的，tag1—-tag50之间，但符合条件的tag最多为5个(不为空只有5个)，转换成行。

root@xxx 08:17:54>select * from test;
+------------+-------+
| a          | b     |
+------------+-------+
| 2011-08-22 | tag1  | 
| 2011-08-22 | tag2  | 
| 2011-08-22 | tag3  | 
| 2011-08-22 | tag30 | 
| 2011-08-22 | tag40 | 
| 2011-08-22 | NULL  | 
+------------+-------+

结果如下：

+------------+-------+-------+-------+-------+-------+ 
| a          | b1    | b2    | b3    | b4    | b5    | 
+------------+-------+-------+-------+-------+-------+ 
| 2011-08-22 | tag1  | tag2  |tag3   |tag30  | tag40 |
+------------+-------+-------+-------+-------+-------+

我的思路：

1、把行的转换成列:

+------------+-------+-------+-------+----|
| a          | b1                         |
+------------+-------+-------+-------+----|
| 2011-08-22 | tag1,tag2,tag3,tag30,tag40 |
+------------+-------+-------+-------+----|

2、再b1字段拆开来，拆成多个字段

解决方法：

root@xxx 08:24:46>select a,group_concat(b) from test where b is not null;
+------------+----------------------------+
| a          | group_concat(b)            |
+------------+----------------------------+
| 2011-08-22 | tag1,tag2,tag3,tag30,tag40 | 
+------------+----------------------------+

分割函数( http://blog.fedecarg.com/2009/02/22/mysql-split-string-function/)：

root@xxx 08:25:58>CREATE FUNCTION SPLIT_STR(
    ->   x VARCHAR(255),
    ->   delim VARCHAR(12),
    ->   pos INT
    -> )
    -> RETURNS VARCHAR(255)
    -> RETURN REPLACE(SUBSTRING(SUBSTRING_INDEX(x, delim, pos),
    ->        LENGTH(SUBSTRING_INDEX(x, delim, pos -1)) + 1),
    ->        delim, '');
Query OK, 0 rows affected (0.00 sec)
 
root@xxx 08:28:11>select a,
    ->        split_str(group_concat(b), ',', 1) b1,
    ->        split_str(group_concat(b), ',', 2) b2,
    ->        split_str(group_concat(b), ',', 3) b3,
    ->        split_str(group_concat(b), ',', 4) b4,
    ->        split_str(group_concat(b), ',', 5) b5
    ->   from test
    ->  where b is not null;
+------------+------+------+------+-------+-------+
| a          | b1   | b2   | b3   | b4    | b5    |
+------------+------+------+------+-------+-------+
| 2011-08-22 | tag1 | tag2 | tag3 | tag30 | tag40 | 
+------------+------+------+------+-------+-------+