MySQL - 如何将列反透视为行?

2021-11-20 00:00:00 sql union mysql unpivot lateral-join

此时我可能看的不是很清楚，但我在 MySQL 中有一个表，如下所示:

I'm probably not seeing things very clear at this moment, but I have a table in MySQL which looks like this:

ID | a  | b  | c 
1  | a1 | b1 | c1
2  | a2 | b2 | c2

出于某种原因(实际上是另一个表上的连接 - 基于 ID，但我认为如果有人可以帮助我完成这部分，我可以自己完成其余部分)，我需要这些行改为这样:

For some reason (actually a join on another table - based on ID, but I think if someone can help me out with this part, I can do the rest myself), I needed those rows to be like this instead:

1 | a1 | a
1 | b1 | b
1 | c1 | c
2 | a2 | a
2 | b2 | b
2 | c2 | c

所以基本上，我需要查看如下行:ID、columntitle、value有什么方法可以轻松做到这一点吗?

So basically, I need to view the rows like: ID, columntitle, value Is there any way to do this easily?

推荐答案

您正在尝试反透视数据.MySQL 没有 unpivot 功能，因此您必须使用 UNION ALL 查询将列转换为行:

You are trying to unpivot the data. MySQL does not have an unpivot function, so you will have to use a UNION ALL query to convert the columns into rows:

select id, 'a' col, a value
from yourtable
union all
select id, 'b' col, b value
from yourtable
union all
select id, 'c' col, c value
from yourtable

参见SQL Fiddle with Demo.

这也可以使用 CROSS JOIN 来完成:

This can also be done using a CROSS JOIN:

select t.id,
  c.col,
  case c.col
    when 'a' then a
    when 'b' then b
    when 'c' then c
  end as data
from yourtable t
cross join
(
  select 'a' as col
  union all select 'b'
  union all select 'c'
) c

参见SQL Fiddle with Demo

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