MySQL - How to unpivot columns to rows?

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

It took a long time coming, but MySQL version 8.0.14 finally added support for lateral joins - the official terminology is lateral derived tables.

This is a very powerful feature, that comes handy in multiple situations, including unpivoting table columns to rows.

You can phrase the query as follows:

select t.id, x.*
from mytable t
cross join lateral (
    select a, 'a' 
    union all select b, 'b'
    union all select c, 'c'
) as x(col1, col2)

It may look like this is not a big difference compared to the typical cannonical solution - after all, we are still using union all within the lateral derived table... But don't get it wrong: this query scans the table only once, as opposed to the other approach, which requires one scan for each column to unpivot. So this is more efficient - and the performance gain increases dramatically as the table goes bigger and/or more columns need to be unpivoted.

Bottom line: if you are running MySQL 8.0.14 or higher, just use this technique. From that version onwards, this is the canonical way to unpivot in MYSQL.

Demo on DB Fiddle:

Sample data:

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

Query results:

id | col1 | col2
-: | :--- | :---
 1 | a1   | a   
 1 | b1   | b   
 1 | c1   | c   
 2 | a2   | a   
 2 | b2   | b   
 2 | c2   | c   

Update

MySQL 8.0.19 first introduced support for the VALUES statement, which could help further shortening the query by removing the need to use union all in a subquery (although I don't see any performance gain here, this makes the query neater). As of version 8.0.30, we can express the lateral join like so:

select t.id, x.*
from mytable t
cross join lateral (values 
    row(a, 'a'), 
    row(b, 'b'),
    row(c, 'c')
) as x(col1, col2);

Try to use UNION ALL.

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