可以将多个行和表的字符串连接到一个结果列中吗？ [英] Is it possible to concatenate strings from multiple rows and tables into one result column?

问题描述

我正在尝试编写一个MySQL查询，从表项目中检索一个与表标签有一对多关系的记录。我的应用程序使用4个表来执行此操作：

 项目 - 项目表
实体 - 实体表;引用几个应用程序资源
标签 - 标签表
 Tag_entity  - 链接标签到实体
  

是否可以以这样的方式编写查询，使表标签中的多个值连接到一个结果列中？我不喜欢这样做，而不使用子查询。

表格澄清：

  ------------- 
 | Tag_Entity | 
 ------------- ---------- | ----------- | ------- 
 |项目| |实体| | -  id | |标签| 
 | ----------- | | -------- | | -  tag_id | | ----- | 
 | -  id | - > | -  id | - > | -  entity_id | - > | id | 
 | -  entity_id | ---------- ------------- |名称| 
 ------------- ------- 
  

期望的结果：

  Projects.id Entities.id Tags.name（concatenated）
 1 5'foo '，'bar'，'etc'
  

解决方案

一个href =http://dev.mysql.com/doc/refman/5.0/en/group-by-functions.html#function_group-concat =noreferrer> GROUP_CONCAT

示例：

  mysql> SELECT * FROM blah; 
 + ---- + ----- + ----------- + 
 | K | grp |名称| 
 + ---- + ----- + ----------- + 
 | 1 | 1 | foo | 
 | 2 | 1 |酒吧| 
 | 3 | 2 |氢| 
 | 4 | 4 | dasher | 
 | 5 | 2 |氦气| 
 | 6 | 2 |锂| 
 | 7 | 4 |舞者| 
 | 8 | 3 | winken | 
 | 9 | 4 | prancer | 
 | 10 | 2 |铍| 
 | 11 | 1 | baz | 
 | 12 | 3 | blinken | 
 | 13 | 4 | vixen | 
 | 14 | 1 | quux | 
 | 15 | 4 |彗星| 
 | 16 | 2 |硼| 
 | 17 | 4 |丘比特| 
 | 18 | 4 | donner | 
 | 19 | 4 | blitzen | 
 | 20 | 3 | nod | 
 | 21 | 4 | rudolph | 
 + ---- + ----- + ----------- + 
 21行集（0.00秒）
 
 mysql> SELECT grp，GROUP_CONCAT（name ORDER BY K）FROM blah GROUP BY grp; 
 + ----- + --------------------------------------- ------------------------- + 
 | grp | GROUP_CONCAT（名称ORDER BY K）| 
 + ----- + --------------------------------------- ------------------------- + 
 | 1 | foo，bar，baz，quux | 
 | 2 |氢，氦，锂，铍，硼| 
 | 3 |眨眼，眨眼，点头
 | 4 | dasher，dancer，prancer，vixen，彗星，丘比特，donner，blitzen，rudolph | 
 + ----- + --------------------------------------- ------------------------- + 
 4行集（0.00秒）
  

I am trying to write a MySQL query that retrieves one record from table "projects" that has a one-to-many relationship with table "tags". My application uses 4 tables to do this:

Projects - the projects table
Entities - entity table; references several application resources
Tags - tags table
Tag_entity - links tags to entities

Is it possible to write the query in such a way that multiple values from table "Tags" are concatenated into one result column? I'd prefer doing this without using subqueries.

Table clarification:

                                      -------------
                                     | Tag_Entity  |
 -------------       ----------      | ----------- |      -------
| Projects    |     | Entities |     | - id        |     | Tags  |
| ----------- |     | -------- |     | - tag_id    |     | ----- |
| - id        | --> | - id     | --> | - entity_id | --> | id    |
| - entity_id |      ----------       -------------      | name  |
 -------------                                            -------

Desired result:

Projects.id   Entities.id   Tags.name (concatenated)
1             5             'foo','bar','etc'

解决方案

see GROUP_CONCAT

example:

mysql> SELECT * FROM blah;
+----+-----+-----------+
| K  | grp | name      |
+----+-----+-----------+
|  1 |   1 | foo       |
|  2 |   1 | bar       |
|  3 |   2 | hydrogen  |
|  4 |   4 | dasher    |
|  5 |   2 | helium    |
|  6 |   2 | lithium   |
|  7 |   4 | dancer    |
|  8 |   3 | winken    |
|  9 |   4 | prancer   |
| 10 |   2 | beryllium |
| 11 |   1 | baz       |
| 12 |   3 | blinken   |
| 13 |   4 | vixen     |
| 14 |   1 | quux      |
| 15 |   4 | comet     |
| 16 |   2 | boron     |
| 17 |   4 | cupid     |
| 18 |   4 | donner    |
| 19 |   4 | blitzen   |
| 20 |   3 | nod       |
| 21 |   4 | rudolph   |
+----+-----+-----------+
21 rows in set (0.00 sec)

mysql> SELECT grp, GROUP_CONCAT(name ORDER BY K) FROM blah GROUP BY grp;
+-----+----------------------------------------------------------------+
| grp | GROUP_CONCAT(name ORDER BY K)                                  |
+-----+----------------------------------------------------------------+
|   1 | foo,bar,baz,quux                                               |
|   2 | hydrogen,helium,lithium,beryllium,boron                        |
|   3 | winken,blinken,nod                                             |
|   4 | dasher,dancer,prancer,vixen,comet,cupid,donner,blitzen,rudolph |
+-----+----------------------------------------------------------------+
4 rows in set (0.00 sec)

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