Transposing data table on SQL [duplicate] - mysql

Question

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?

Answer 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

Answer 2

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
Side note
MySQL 8.0.19 added 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).
Unfortunately, as of version 8.0.21, this does not work yet - which might be considered a bug - but maybe will in a future version...:
select t.id, x.*
from mytable t
cross join lateral (values
row(a, 'a'),
row(b, 'b'),
row(c, 'c')
) as x(col1, col2);

Answer 3

Try to use UNION ALL.
SELECT ID, a, 'a'
FROM tbl
WHERE ID = 1
UNION
SELECT ID, b, 'b'
FROM tbl
WHERE ID = 2