MYSQL函數group_concat的使用

今天對一批數據要遷移轉換，查了下，有group_concat這個函數簡單實現字段的列轉行設置，過程記錄以下

一.測試數據準備

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

二.使用通過
1.以默認的逗號做爲分隔符

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

2.對ID值進行排序後行轉列

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.使用其餘分割符，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.分隔符與排序結合起來用

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.對相同的值分組

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)

三.參數設置與限制說明
1.查看服務器中設置

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

以上設置的值說明當前是默認長度1KB  2.改變參數值 方法一：修改配置文件中參數，新增 group_concat_max_len = 10240 方法二：在會話中實現，全局或當前session中 SET GLOBAL group_concat_max_len=10240; SET SESSION group_concat_max_len=10240;