unpivot and PostgreSQL

Question

Is there a unpivot equivalent function in PostgreSQL?

Answer 1

Create an example table:

CREATE TEMP TABLE foo (id int, a text, b text, c text);
INSERT INTO foo VALUES (1, 'ant', 'cat', 'chimp'), (2, 'grape', 'mint', 'basil');

You can 'unpivot' or 'uncrosstab' using UNION ALL:

SELECT id,
       'a' AS colname,
       a AS thing
FROM foo
UNION ALL
SELECT id,
       'b' AS colname, 
       b AS thing
FROM foo
UNION ALL
SELECT id, 
       'c' AS colname,
       c AS thing
FROM foo
ORDER BY id;

This runs 3 different subqueries on foo, one for each column we want to unpivot, and returns, in one table, every record from each of the subqueries.

But that will scan the table N times, where N is the number of columns you want to unpivot. This is inefficient, and a big problem when, for example, you're working with a very large table that takes a long time to scan.

Instead, use:

SELECT id,
       unnest(array['a', 'b', 'c']) AS colname,
       unnest(array[a, b, c]) AS thing
FROM foo
ORDER BY id;

This is easier to write, and it will only scan the table once.

array[a, b, c] returns an array object, with the values of a, b, and c as it's elements. unnest(array[a, b, c]) breaks the results into one row for each of the array's elements.

Hope that helps!

Answer 2

You could use VALUES() and JOIN LATERAL to unpivot the columns.

Sample data:

CREATE TABLE test(id int, a INT, b INT, c INT);
INSERT INTO test(id,a,b,c) VALUES (1,11,12,13),(2,21,22,23),(3,31,32,33);

Query:

SELECT t.id, s.col_name, s.col_value
FROM test t
JOIN LATERAL(VALUES('a',t.a),('b',t.b),('c',t.c)) s(col_name, col_value) ON TRUE;

DBFiddle Demo

Using this approach it is possible to unpivot multiple groups of columns at once.

EDIT

Using Zack's suggestion:

SELECT t.id, col_name, col_value
FROM test t
CROSS JOIN LATERAL (VALUES('a', t.a),('b', t.b),('c',t.c)) s(col_name, col_value);

<=>

SELECT t.id, col_name, col_value
FROM test t
,LATERAL (VALUES('a', t.a),('b', t.b),('c',t.c)) s(col_name, col_value);

db<>fiddle demo

Answer 3

FYI for those of us looking for how to unpivot in RedShift.

The long form solution given by Stew appears to be the only way to accomplish this.

https://forums.aws.amazon.com/thread.jspa?threadID=126369

---

For those who cannot see it there here is the text pasted below...

We do not have built-in functions that will do pivot or unpivot. However, you can always write SQL to do that.

create table sales (regionid integer, q1 integer, q2 integer, q3 integer, q4 integer);
insert into sales values (1,10,12,14,16), (2,20,22,24,26);

select * from sales order by regionid;

regionid | q1 | q2 | q3 | q4
----------+----+----+----+----
1 | 10 | 12 | 14 | 16
2 | 20 | 22 | 24 | 26

(2 rows)

pivot query

create table sales_pivoted (regionid, quarter, sales)
as
select regionid, 'Q1', q1 from sales
UNION ALL
select regionid, 'Q2', q2 from sales
UNION ALL
select regionid, 'Q3', q3 from sales
UNION ALL
select regionid, 'Q4', q4 from sales
;

select * from sales_pivoted order by regionid, quarter;

regionid | quarter | sales 
----------+---------+-------
1 | Q1 | 10
1 | Q2 | 12
1 | Q3 | 14
1 | Q4 | 16
2 | Q1 | 20
2 | Q2 | 22
2 | Q3 | 24
2 | Q4 | 26
(8 rows)

unpivot query

select regionid, sum(Q1) as Q1, sum(Q2) as Q2, sum(Q3) as Q3, sum(Q4) as Q4
from
(select regionid, 
case quarter when 'Q1' then sales else 0 end as Q1,
case quarter when 'Q2' then sales else 0 end as Q2,
case quarter when 'Q3' then sales else 0 end as Q3,
case quarter when 'Q4' then sales else 0 end as Q4
from sales_pivoted)

group by regionid
order by regionid;
regionid | q1 | q2 | q3 | q4 
----------+----+----+----+----
1 | 10 | 12 | 14 | 16
2 | 20 | 22 | 24 | 26
(2 rows)

Hope this helps, Neil

Answer 4

I wrote a horrible unpivot function for PostgreSQL. It's rather slow but it at least returns results like you'd expect an unpivot operation to.

https://cgsrv1.arrc.csiro.au/blog/2010/05/14/unpivotuncrosstab-in-postgresql/

Hopefully you can find it useful..

Answer 5

Pulling slightly modified content from the link in the comment from @a_horse_with_no_name into an answer because it works:

Installing Hstore If you don't have hstore installed and are running PostgreSQL 9.1+, you can use the handy

CREATE EXTENSION hstore;

For lower versions, look for the hstore.sql file in share/contrib and run in your database.

Assuming that your source (e.g., wide data) table has one 'id' column, named id_field, and any number of 'value' columns, all of the same type, the following will create an unpivoted view of that table.

CREATE VIEW vw_unpivot 
AS 
SELECT id_field, (h).key AS column_name, (h).value AS column_value
  FROM (SELECT id_field, each(hstore(foo) - 'id_field'::text) AS h
          FROM zcta5 as foo  
       ) AS unpiv ; 

This works with any number of 'value' columns. All of the resulting values will be text, unless you cast, e.g., (h).value::numeric.