MySQL - turn table into different table

问题:

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

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

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

回答1:

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

See SQL Fiddle with Demo.

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

See SQL Fiddle with Demo

回答2:

Try to use UNION ALL.

SELECT ID, a, \'a\' 
FROM tbl
WHERE ID = 1
UNION
SELECT ID, b, \'b\' 
FROM tbl
WHERE ID = 2