MySQL - How to convert columns to rows?

Question

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

How to reorganize rows into IDs, column headers, values?

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

Answer 1

You are trying to reverse the data. MySQL does not have a reverse function, so you need to use a UNION ALL query to convert columns to 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

See SQL Fiddle with Demo.

This can also be achieved using 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

See SQL Fiddle with Demo