MySQL group_concat

group_concat()会计算哪些行属于同一组，将属于同一组的列显示出来,实现字段的列转行设置

语法：

GROUP_CONCAT([DISTINCT] expr [,expr ...]
[ORDER BY {unsigned_integer | col_name | expr}
[ASC | DESC] [,col_name ...]]
[SEPARATOR str_val])

mysql> select * from t_test;
+------+
| id   |
+------+
|    1 |
|    2 |
|    3 |
|    4 |
|    5 |
+------+
5 rows in set (0.00 sec)

以默认的逗号作为分隔符

mysql> select group_concat(id) from t_test;
+------------------+
| group_concat(id) |
+------------------+
| 1,2,3,4,5 |
+------------------+
1 row in set (0.00 sec)

order

mysql> select id,group_concat(name order by name desc) from  t_test group by id;

separator

SELECT id,group_concat(name,separator ';') groups FROM t_test group by id;
+----+------------+
| id |  groups    |
+-------+---------+
| 1 |  tom;Jim    |
| 2 |  hulu       |
| 3 |  Jack;opt   |
| 4 |             |
| 5 |  Sam;phony;childish |
+---+---------------------+

注意：默认下，group_concat()长度为1024，

group_concat截取的结果 会发现 缺失，

修改参数方法：

set global group_concat_max_len = 102400；

或者修改 my.ini 中group_concat_max_len