MYSQL函数group_concat的使用

今天对一批数据要迁移转换，查了下，有group_concat这个函数简单实现字段的列转行设置，过程记录如下 

一.测试数据准备

mysql> use test;
Database changed
mysql> select * from t_kenyon;
+------+
| id   |
+------+
|    1 |
|  123 |
|  789 |
|  345 |
|   78 |
+------+
5 rows in set (0.00 sec)

二.使用经过  
1.以默认的逗号作为分隔符

mysql> select group_concat(id) from t_kenyon;
+------------------+
| group_concat(id) |
+------------------+
| 1,123,789,345,78 |
+------------------+
1 row in set (0.00 sec)

2.对ID值进行排序后行转列

mysql> select group_concat(id order by id) from t_kenyon;
+------------------------------+
| group_concat(id order by id) |
+------------------------------+
| 1,78,123,345,789             |
+------------------------------+
1 row in set (0.00 sec)

3.使用其他分割符，如*和;等

mysql> select group_concat(id separator '*') from t_kenyon;
+--------------------------------+
| group_concat(id separator '*') |
+--------------------------------+
| 1*123*789*345*78               |
+--------------------------------+
1 row in set (0.00 sec)

4.分隔符与排序结合起来用

mysql> select group_concat(id order by id separator '_') from t_kenyon;
+--------------------------------------------+
| group_concat(id order by id separator '_') |
+--------------------------------------------+
| 1_78_123_345_789                           |
+--------------------------------------------+
1 row in set (0.00 sec)

5.对相同的值分组

mysql> insert into t_kenyon values (78);
Query OK, 1 row affected (0.00 sec)mysql> select group_concat(id) from t_kenyon group by id;
+------------------+
| group_concat(id) |
+------------------+
| 1                |
| 78,78            |
| 123              |
| 345              |
| 789              |
+------------------+
5 rows in set (0.00 sec)

三.参数设置与限制说明 
1.查看服务器中设置

mysql> show variables like '%group_concat%';
+----------------------+-------+
| Variable_name        | Value |
+----------------------+-------+
| group_concat_max_len | 1024  |
+----------------------+-------+
1 row in set (0.00 sec)

以上设置的值说明当前是默认长度1KB 

 2.改变参数值 
方法一：修改配置文件中参数，新增 group_concat_max_len = 10240 
方法二：在会话中实现，全局或当前session中 
SET GLOBAL group_concat_max_len=10240; 
SET SESSION group_concat_max_len=10240;