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多个值，并且在所有连接之后，我得到的行数比我想要的多得多。
我在MySQL Doc上寻找了一个函数，它看起来不像CONCATorCONCAT_WS函数接受结果集。
那么这里有人知道怎么做吗？
您可以使用GROUP_CONCAT：
SELECT person_id,
GROUP_CONCAT(hobbies SEPARATOR ', ')
FROM peoples_hobbies
GROUP BY person_id;
正如路德维希在他的评论中所说，您可以添加DISTINCT运算符以避免重复：
SELECT person_id,
GROUP_CONCAT(DISTINCT hobbies SEPARATOR ', ')
FROM peoples_hobbies
GROUP BY person_id;
正如 Jan 在他们的评论中所说，您还可以在使用以下方法对值进行排序之前对其进行排序ORDER BY：
SELECT person_id,
GROUP_CONCAT(hobbies ORDER BY hobbies ASC SEPARATOR ', ')
FROM peoples_hobbies
GROUP BY person_id;
正如 Dag 在他的评论中所说，结果有 1024 字节的限制。要解决此问题，请在查询之前运行此查询：
SET group_concat_max_len = 2048;
当然，您可以2048根据自己的需要进行更改。要计算和分配值：
SET group_concat_max_len = CAST(
(SELECT SUM(LENGTH(hobbies)) + COUNT(*) * LENGTH(', ')
FROM peoples_hobbies
GROUP BY person_id) AS UNSIGNED);
看看GROUP_CONCAT你的 MySQL 版本 (4.1) 是否支持它。有关更多详细信息，请参阅文档。
它看起来像：
SELECT GROUP_CONCAT(hobbies SEPARATOR ', ')
FROM peoples_hobbies
WHERE person_id = 5
GROUP BY 'all';
警告：这篇文章会让你感到饥饿。
我发现自己想要选择多个单独的行——而不是一个组——并在某个字段上连接。
假设您有一张包含产品 ID 及其名称和价格的表格：
+------------+--------------------+-------+
| product_id | name | price |
+------------+--------------------+-------+
| 13 | Double Double | 5 |
| 14 | Neapolitan Shake | 2 |
| 15 | Animal Style Fries | 3 |
| 16 | Root Beer | 2 |
| 17 | Lame T-Shirt | 15 |
+------------+--------------------+-------+
然后你有一些花哨的 ajax 将这些小狗列为复选框。
您的饥饿河马用户选择13, 15, 16. 今天没有给她甜点...
一种使用纯 mysql 在一行中总结用户订单的方法。
mysql> SELECT GROUP_CONCAT(name SEPARATOR ' + ') AS order_summary FROM product WHERE product_id IN (13, 15, 16);
哪个输出：
+------------------------------------------------+
| order_summary |
+------------------------------------------------+
| Double Double + Animal Style Fries + Root Beer |
+------------------------------------------------+
如果您也想要总价，请输入SUM()：
mysql> SELECT GROUP_CONCAT(name SEPARATOR ' + ') AS order_summary, SUM(price) AS total FROM product WHERE product_id IN (13, 15, 16);
+------------------------------------------------+-------+
| order_summary | total |
+------------------------------------------------+-------+
| Double Double + Animal Style Fries + Root Beer | 10 |
+------------------------------------------------+-------+
GROUP_CONCAT您可以通过设置group_concat_max_len参数来更改值的最大长度。
请参阅MySQL 文档中的详细信息。
有一个 GROUP 聚合函数GROUP_CONCAT。
就我而言，我有一排 Id，必须将其转换为 char，否则，结果将被编码为二进制格式：
SELECT CAST(GROUP_CONCAT(field SEPARATOR ',') AS CHAR) FROM table
使用 MySQL(5.6.13) 会话变量和赋值运算符，如下所示
SELECT @logmsg := CONCAT_ws(',',@logmsg,items) FROM temp_SplitFields a;
那么你可以得到
test1,test11
我有一个更复杂的查询，发现我必须GROUP_CONCAT在外部查询中使用才能让它工作：
SELECT DISTINCT userID
FROM event GROUP BY userID
HAVING count(distinct(cohort))=2);
SELECT GROUP_CONCAT(sub.userID SEPARATOR ', ')
FROM (SELECT DISTINCT userID FROM event
GROUP BY userID HAVING count(distinct(cohort))=2) as sub;
希望这可能对某人有所帮助。
对于在这里查看如何使用GROUP_CONCAT子查询的人-发布此示例
SELECT i.*,
(SELECT GROUP_CONCAT(userid) FROM favourites f WHERE f.itemid = i.id) AS idlist
FROM items i
WHERE i.id = $someid
所以GROUP_CONCAT必须在子查询中使用，而不是包装它。
试试这个：
DECLARE @Hobbies NVARCHAR(200) = ' '
SELECT @Hobbies = @Hobbies + hobbies + ',' FROM peoples_hobbies WHERE person_id = 5;
TL;博士;
set @sql='';
set @result='';
set @separator=' union \r\n';
SELECT
@sql:=concat('select ''',INFORMATION_SCHEMA.COLUMNS.COLUMN_NAME ,''' as col_name,',
INFORMATION_SCHEMA.COLUMNS.CHARACTER_MAXIMUM_LENGTH ,' as def_len ,' ,
'MAX(CHAR_LENGTH(',INFORMATION_SCHEMA.COLUMNS.COLUMN_NAME , '))as max_char_len',
' FROM ',
INFORMATION_SCHEMA.COLUMNS.TABLE_NAME
) as sql_piece, if(@result:=if(@result='',@sql,concat(@result,@separator,@sql)),'','') as dummy
FROM INFORMATION_SCHEMA.COLUMNS
WHERE
INFORMATION_SCHEMA.COLUMNS.DATA_TYPE like '%char%'
and INFORMATION_SCHEMA.COLUMNS.TABLE_SCHEMA='xxx'
and INFORMATION_SCHEMA.COLUMNS.TABLE_NAME='yyy';
select @result;
我们有两种方法来连接 MySql 中的列
select concat(hobbies) as `Hobbies` from people_hobbies where 1
或者
select group_concat(hobbies) as `Hobbies` from people_hobbies where 1
已经很晚了，但对于那些正在搜索“使用数据透视表将多个 MySQL 行连接到一个字段”的人会有所帮助:)
询问：
SELECT pm.id, pm.name, GROUP_CONCAT(c.name) as channel_names
FROM payment_methods pm
LEFT JOIN payment_methods_channels_pivot pmcp ON pmcp.payment_method_id = pm.id
LEFT JOIN channels c ON c.id = pmcp.channel_id
GROUP BY pm.id
表
payment_methods
id | name
1 | PayPal
channels
id | name
1 | Google
2 | Faceook
payment_methods_channels_pivot
payment_method_id | channel_id
1 | 1
1 | 2
输出：
在 sql server 中使用 string_agg 将行字段值转换为列：
select string_agg(field1, ', ') a FROM mytable
or
select string_agg(field1, ', ') within group (order by field1 dsc) a FROM mytable group by field2
在这里，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
这里
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排在第三位。所以我使用最后一行
Order by Id DESC
LIMIT 1
然后我还要从我的字符串中删除第一个'，'
TRIM(LEADING ', ' FROM T.hobbies )
在这种情况下另一个有趣的例子 -
以下是示例表的结构people_hobbies-
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 | |
+---------+--------------+------+-----+---------+----------------+
该表填充如下 -
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，hobby_list生成了一个表格，其中包含所有人的列表和每个人的爱好列表，每个爱好在一个新行中 -
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;
