我可以将多个MySQL行连接成一个字段吗？

您可以使用GROUP_CONCAT：

SELECT person_id,
   GROUP_CONCAT(hobbies SEPARATOR ', ')
FROM peoples_hobbies
GROUP BY person_id;

正如 Ludwig 在他的评论中所说，您可以添加运算符以避免重复：DISTINCT

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，您还可以在内爆之前使用以下方法对值进行排序：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。若要解决此问题，请在查询之前运行此查询：

SET group_concat_max_len = 2048;

当然，您可以根据需要进行更改。要计算和赋值，请执行以下操作：2048

SET group_concat_max_len = CAST(
                     (SELECT SUM(LENGTH(hobbies)) + COUNT(*) * LENGTH(', ')
                           FROM peoples_hobbies
                           GROUP BY person_id) AS UNSIGNED);

GROUP_CONCAT，有一个 GROUP Aggregate 函数。

看看你的MySQL版本（4.1）是否支持它。有关更多详细信息，请参阅文档。GROUP_CONCAT

它看起来像这样：

  SELECT GROUP_CONCAT(hobbies SEPARATOR ', ') 
  FROM peoples_hobbies 
  WHERE person_id = 5 
  GROUP BY 'all';

您可以通过设置参数来更改值的最大长度。GROUP_CONCATgroup_concat_max_len

有关详细信息，请参阅 MySQL 文档。

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;

然后你可以得到

test1,test11

就我而言，我有一行 Id，必须将其转换为 char，否则，结果被编码为二进制格式：

SELECT CAST(GROUP_CONCAT(field SEPARATOR ',') AS CHAR) FROM table

我发现自己想要选择多个单独的行（而不是一组），并在某个字段上连接。

鉴于：

假设您有一个产品 ID 及其名称和价格表：

+------------+--------------------+-------+
| 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。

饥饿的河马用户选择 。今天没有给她吃甜点......13, 15, 16

找到：

一种用纯 mysql 在一行中总结用户订单的方法。

溶液：

与 IN 子句一起使用：GROUP_CONCAT

mysql> SELECT GROUP_CONCAT(name SEPARATOR ' + ') AS order_summary FROM product WHERE product_id IN (13, 15, 16);

输出：

+------------------------------------------------+
| order_summary                                  |
+------------------------------------------------+
| Double Double + Animal Style Fries + Root Beer |
+------------------------------------------------+

奖金解决方案：

如果你也想要总价，请抛入 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 |
+------------------------------------------------+-------+

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;

我有一个更复杂的查询，发现我必须在外部查询中使用才能使其工作：GROUP_CONCAT

原始查询：

SELECT DISTINCT userID 
FROM event GROUP BY userID 
HAVING count(distinct(cohort))=2);

内爆：

SELECT GROUP_CONCAT(sub.userID SEPARATOR ', ') 
FROM (SELECT DISTINCT userID FROM event 
GROUP BY userID HAVING count(distinct(cohort))=2) as sub;

希望这可能会对某人有所帮助。

我们有两种方法可以在 MySql 中连接列

select concat(hobbies) as `Hobbies` from people_hobbies where 1

或

select group_concat(hobbies) as `Hobbies` from people_hobbies where 1

对于在这里寻找如何使用子查询的人 - 发布此示例GROUP_CONCAT

SELECT i.*,
(SELECT GROUP_CONCAT(userid) FROM favourites f WHERE f.itemid = i.id) AS idlist
FROM items i
WHERE i.id = $someid

所以必须在子查询中使用，而不是包装它。GROUP_CONCAT

虽然很晚，但对于那些正在搜索“使用数据透视表将多个MySQL行连接成一个字段”的人来说会有所帮助:)

查询：

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 将行字段值透视到列中：

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，每个爱好都在一个新行中 -hobby_list

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;

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

会回来

  Id    hobbies
  1     , shopping
  2     , shopping, fishing
  3     , shopping, fishing, coding

现在我们的预期成绩是排在第三位。所以我使用

  Order by Id DESC 
  LIMIT 1
  

然后我也从我的字符串中删除了第一个'，'

 TRIM(LEADING ', ' FROM T.hobbies )

使用GROUP_CONCAT：

SELECT GROUP_CONCAT(hobbies) FROM peoples_hobbies WHERE person_id = 5;