MySQL - How to unpivot columns to rows?
Solution 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
Solution 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
Solution 3
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.
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).
nfortunately, 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);
user2128539
Updated on October 18, 2020Comments
-
user2128539 over 3 years
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? -
user2128539 over 11 yearsThank you. So it seems there wasn't a real shorter way after all (it's quite a few columns, I thought there would be an easier way than a 'select ... from ... ' for every column.
-
Taryn over 11 years@user2128539 see my edit, I included a different way to unpivot the data.
-
Barmar almost 4 yearsGet rid of the
WHERE
clauses. Otherwise you're only showing one column for each ID, instead of all columns for each ID. -
Barmar almost 4 years@user2128539 You should accept the answer that solved the problem.
-
Darshan Jain about 3 yearsLife saver answer