mysql - 将列转换为行

Question

我有一张 table :

+--------------+-------+--------+----------+
| attribute_id | color | brand  | category |
+--------------+-------+--------+----------+
|            1 | red   | honda  | cars     |
|            2 | blue  | bmw    | cars     |
|            3 | pink  | skonda | vans     |
+--------------+-------+--------+----------+

我想将其转换为以下内容:

+--------------+---------+
| attribute_id | keyword |
+--------------+---------+
|            1 | red     |
|            2 | blue    |
|            3 | pink    |
|            1 | honda   |
|            2 | bmw     |
|            3 | skonda  |
|            1 | cars    |
|            2 | cars    |
|            3 | vans    |
+--------------+---------+

我能想到的唯一方法是像这样使用 UNION:

SELECT attribute_id, color from attributes 
UNION ALL
SELECT attribute_id, brand from attributes
UNION ALL
SELECT attribute_id, category from attributes

上面的方式有点麻烦，特别是因为我的实际用例需要为每个选择连接多个表。

是否有更简单或更少的复制/粘贴方式来编写此代码？

Answer 1

一个更有效的查询(至少对于大表)是:

SELECT attribute_id,
       (case when n = 1 then color
             when n = 2 then brand
             when n = 3 then category
        end) as keyword
from attributes a cross join
     (select 1 as n union all select 2 union all select 3) n;

这比 union all 查询更好的原因是性能。 union all 将扫描原始表三次。这将扫描原始表一次(然后循环遍历 n)。对于大型表，这可能会在性能上产生显着差异。