MySQL - 如何将列转换为行？

Question

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

如何将行重新组织为ID，列标题，值？

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

Answer 1

您正在尝试对数据进行反转。MySQL没有反转函数，因此您需要使用UNION ALL查询将列转换为行：

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

See SQL Fiddle with Demo.

这也可以使用CROSS JOIN来实现：

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

See SQL Fiddle with Demo