使用MySQL的数据透视表

Question

我有两个表三元组和标签

三元组表具有以下列

  id  PostID  TagID   Value
   1   1        1     Murder
   2   1        2     New Brunswick
   3   2        1     Theft
   4   2        3     Gun 

标签表具有以下列

  id   TagName
   1   Incident
   2   Location
   3   Weapon    

我正在尝试编写SQL来创建带有动态标头的数据透视表

输出应该是这样的

       PostID  Incident   Location         Weapon        
           1   Murder     New Brunswick    
           2   Theft                        Gun 

任何写SQL的帮助将不胜感激.我在网上看到了一些例子但却无法弄清楚这一点

Answer 1

为了在MySQL中转动数据,您需要同时使用聚合函数和CASE表达式.

如果您有已知数量的列,则可以对查询进行硬编码:

select p.postid,
  max(case when t.tagname = 'Incident' then p.value end) Incident,
  max(case when t.tagname = 'Location' then p.value end) Location,
  max(case when t.tagname = 'Weapon' then p.value end) Weapon
from triples p
left join tags t
  on p.tagid = t.id
group by p.postid;

请参阅SQL Fiddle with Demo

但是如果您有未知数量的列,那么您将需要使用预准备语句来生成动态SQL:

SET @sql = NULL;
SELECT
  GROUP_CONCAT(DISTINCT
    CONCAT(
      'max(CASE WHEN TagName = ''',
      TagName,
      ''' THEN p.value END) AS `',
      TagName, '`'
    )
  ) INTO @sql
FROM tags;


SET @sql 
  = CONCAT('SELECT p.postid, ', @sql, ' 
           from triples p
           left join tags t
             on p.tagid = t.id
           group by p.postid');

PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;

请参阅SQL Fiddle with Demo.

两者都会给出结果:

| POSTID | INCIDENT |      LOCATION | WEAPON |
----------------------------------------------
|      1 |   Murder | New Brunswick | (null) |
|      2 |    Theft |        (null) |    Gun |