问题描述
此时我可能还不太清楚，但我在 MySQL 中有一个如下所示的表:
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(实际上是在另一个表上加入 - 基于 ID，但我认为如果有人可以帮助我完成这部分，我可以自己完成剩下的工作)，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?
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unpivot数据.MySQL 没有 unpivot 函数，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
请参阅 SQL Fiddle with Demo
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