MySQL中group_concat函数
完整的语法如下：
group_concat([DISTINCT]
要连接的字段 [Order BY ASC/DESC 排序字段] [Separator '分隔符'])

基本查询 
mysql>
select * from aa;
+------+------+
| id| name |
+------+------+
|1 |
10|
|1 | 20|
|1 | 20|
|2 | 20|
|3 | 200 |
|3 | 500
|
+------+------+
6 rows in set (0.00 sec)

以id分组，把name字段的值打印在一行，逗号分隔(默认) 
mysql>
select id,group_concat(name) from aa group by
id;
+------+--------------------+
| id| group_concat(name)
|
+------+--------------------+
|1 | 10,20,20|
|2 | 20 |
|3 |
200,500|
+------+--------------------+
3 rows in set (0.00 sec)

以id分组，把name字段的值打印在一行，分号分隔 
mysql>
select id,group_concat(name separator ';') from aa group by
id;
+------+----------------------------------+
| id| group_concat(name
separator ';') |
+------+----------------------------------+
|1 | 10;20;20
|
|2 | 20|
|3 | 200;500
|
+------+----------------------------------+
3 rows in set (0.00 sec)

以id分组，把去冗余的name字段的值打印在一行， 
逗号分隔

mysql>
select id,group_concat(distinct name) from aa group by
id;
+------+-----------------------------+
| id| group_concat(distinct
name) |
+------+-----------------------------+
|1 | 10,20|
|2 | 20
|
|3 | 200,500 |
+------+-----------------------------+
3 rows in set
(0.00 sec)

以id分组，把name字段的值打印在一行，逗号分隔，以name排倒序 
mysql>
select id,group_concat(name order by name desc) from aa group by
id;
+------+---------------------------------------+
| id|
group_concat(name order by name desc)
|
+------+---------------------------------------+
|1 | 20,20,10 |
|2 |
20|
|3 | 500,200|
+------+---------------------------------------+
3
rows in set (0.00 sec)