函数语法：
group_concat( [DISTINCT] 要连接的字段 [Order BY 排序字段 ASC/DESC] [Separator '分隔符'] )
下面举例说明：
select * from goods;
+------+------+
| id| price|
+------+------+
|1 | 10|
|1 | 20|
|1 | 20|
|2 | 20|
|3 | 200 |
|3 | 500 |
+------+------+
6 rows in set (0.00 sec)
以id分组，把price字段的值在同一行打印出来，逗号分隔(默认)
select id, group_concat(price) from goods group by id;
+------+--------------------+
| id| group_concat(price) |
+------+--------------------+
|1 | 10,20,20|
|2 | 20 |
|3 | 200,500|
+------+--------------------+
3 rows in set (0.00 sec)
以id分组，把price字段的值在一行打印出来，分号分隔
select id,group_concat(price separator ';') from goods group by id;
+------+----------------------------------+
| id| group_concat(price separator ';') |
+------+----------------------------------+
|1 | 10;20;20 |
|2 | 20|
|3 | 200;500 |
+------+----------------------------------+
3 rows in set (0.00 sec)
以id分组，把去除重复冗余的price字段的值打印在一行，逗号分隔
select id,group_concat(distinct price) from goods group by id;
+------+-----------------------------+
| id| group_concat(distinct price) |
+------+-----------------------------+
|1 | 10,20|
|2 | 20 |
|3 | 200,500 |
+------+-----------------------------+
3 rows in set (0.00 sec)
以id分组，把price字段的值打印在一行，逗号分隔，按照price倒序排列
select id,group_concat(price order by price desc) from goods group by id;
+------+---------------------------------------+
| id| group_concat(price order by price desc) |
+------+---------------------------------------+
|1 | 20,20,10 |
|2 | 20|
|3 | 500,200|
+------+---------------------------------------+
3 rows in set (0.00 sec)
