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 | +---

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 &＃39;*&＃39;) from t_kenyon;
+--------------------------------+
| group_concat(id separator &＃39;*&＃39;) |
+--------------------------------+
| 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 &＃39;_&＃39;) from t_kenyon;
+--------------------------------------------+
| group_concat(id order by id separator &＃39;_&＃39;) |
+--------------------------------------------+
| 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 &＃39;%group_concat%&＃39;;
+----------------------+-------+
| 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;