使用MySQL,我可以执行以下操作:
SELECT hobbies FROM peoples_hobbies WHERE person_id = 5;
我的输出:
shopping
fishing
coding
但我只想要1行1列:
预期输出:
shopping, fishing, coding
原因是我从多个表中选择了多个值,在所有的连接之后,我得到了比我想要的多得多的行。
我在MySQL Doc上查找了一个函数,它看起来不像CONCAT或CONCAT_WS函数接受结果集。
这里有人知道怎么做吗?
本例中另一个有趣的例子-
下面是people_hobbies示例表的结构-
DESCRIBE people_hobbies;
+---------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+----------------+
| id | int unsigned | NO | PRI | NULL | auto_increment |
| ppl_id | int unsigned | YES | MUL | NULL | |
| name | varchar(200) | YES | | NULL | |
| hby_id | int unsigned | YES | MUL | NULL | |
| hobbies | varchar(50) | YES | | NULL | |
+---------+--------------+------+-----+---------+----------------+
该表的填充方式如下-
SELECT * FROM people_hobbies;
+----+--------+-----------------+--------+-----------+
| id | ppl_id | name | hby_id | hobbies |
+----+--------+-----------------+--------+-----------+
| 1 | 1 | Shriya Jain | 1 | reading |
| 2 | 4 | Shirley Setia | 4 | coding |
| 3 | 2 | Varsha Tripathi | 7 | gardening |
| 4 | 3 | Diya Ghosh | 2 | fishing |
| 5 | 4 | Shirley Setia | 3 | gaming |
| 6 | 1 | Shriya Jain | 6 | cycling |
| 7 | 2 | Varsha Tripathi | 1 | reading |
| 8 | 3 | Diya Ghosh | 5 | shopping |
| 9 | 3 | Diya Ghosh | 4 | coding |
| 10 | 4 | Shirley Setia | 1 | reading |
| 11 | 1 | Shriya Jain | 4 | coding |
| 12 | 1 | Shriya Jain | 3 | gaming |
| 13 | 4 | Shirley Setia | 2 | fishing |
| 14 | 4 | Shirley Setia | 7 | gardening |
| 15 | 2 | Varsha Tripathi | 3 | gaming |
| 16 | 2 | Varsha Tripathi | 2 | fishing |
| 17 | 1 | Shriya Jain | 5 | shopping |
| 18 | 1 | Shriya Jain | 7 | gardening |
| 19 | 3 | Diya Ghosh | 1 | reading |
| 20 | 4 | Shirley Setia | 5 | shopping |
+----+--------+-----------------+--------+-----------+
现在,生成了一个表hobby_list,其中包含所有人的列表和每个人的爱好列表,每个爱好在一行中-
CREATE TABLE hobby_list AS
-> SELECT ppl_id, name,
-> GROUP_CONCAT(hobbies ORDER BY hby_id SEPARATOR "\n")
-> AS hobbies
-> FROM people_hobbies
-> GROUP BY ppl_id
-> ORDER BY ppl_id;
SELECT * FROM hobby_list;
这里,我的意图是在不使用group_concat()函数的情况下应用字符串连接:
Set @concatHobbies = '';
SELECT TRIM(LEADING ', ' FROM T.hobbies ) FROM
(
select
Id, @concatHobbies := concat_ws(', ',@concatHobbies,hobbies) as hobbies
from peoples_hobbies
)T
Order by Id DESC
LIMIT 1
Here
select
Id, @concatHobbies := concat_ws(', ',@concatHobbies,hobbies) as hobbies
from peoples_hobbies
将返回
Id hobbies
1 , shopping
2 , shopping, fishing
3 , shopping, fishing, coding
现在我们的预期结果是第三。所以我用
Order by Id DESC
LIMIT 1
然后我也将第一个“,”从字符串中删除
TRIM(LEADING ', ' FROM T.hobbies )