Postgresql - how to groups rows and convert some rows to columns [duplicate] - postgresql

Question

I have an interesting conundrum which I believe can be solved in purely SQL. I have tables similar to the following:
responses:
user_id | question_id | body
----------------------------
1 | 1 | Yes
2 | 1 | Yes
1 | 2 | Yes
2 | 2 | No
1 | 3 | No
2 | 3 | No
questions:
id | body
-------------------------
1 | Do you like apples?
2 | Do you like oranges?
3 | Do you like carrots?
and I would like to get the following output
user_id | Do you like apples? | Do you like oranges? | Do you like carrots?
---------------------------------------------------------------------------
1 | Yes | Yes | No
2 | Yes | No | No
I don't know how many questions there will be, and they will be dynamic, so I can't just code for every question. I am using PostgreSQL and I believe this is called transposition, but I can't seem to find anything that says the standard way of doing this in SQL. I remember doing this in my database class back in college, but it was in MySQL and I honestly don't remember how we did it.
I'm assuming it will be a combination of joins and a GROUP BY statement, but I can't even figure out how to start.
Anybody know how to do this? Thanks very much!
Edit 1: I found some information about using a crosstab which seems to be what I want, but I'm having trouble making sense of it. Links to better articles would be greatly appreciated!

Answer 1

Use:
SELECT r.user_id,
MAX(CASE WHEN r.question_id = 1 THEN r.body ELSE NULL END) AS "Do you like apples?",
MAX(CASE WHEN r.question_id = 2 THEN r.body ELSE NULL END) AS "Do you like oranges?",
MAX(CASE WHEN r.question_id = 3 THEN r.body ELSE NULL END) AS "Do you like carrots?"
FROM RESPONSES r
JOIN QUESTIONS q ON q.id = r.question_id
GROUP BY r.user_id
This is a standard pivot query, because you are "pivoting" the data from rows to columnar data.

Answer 2

I implemented a truly dynamic function to handle this problem without having to hard code any specific class of answers or use external modules/extensions. It also gives full control over column ordering and supports multiple key and class/attribute columns.
You can find it here: https://github.com/jumpstarter-io/colpivot
Example that solves this particular problem:
begin;
create temporary table responses (
user_id integer,
question_id integer,
body text
) on commit drop;
create temporary table questions (
id integer,
body text
) on commit drop;
insert into responses values (1,1,'Yes'), (2,1,'Yes'), (1,2,'Yes'), (2,2,'No'), (1,3,'No'), (2,3,'No');
insert into questions values (1, 'Do you like apples?'), (2, 'Do you like oranges?'), (3, 'Do you like carrots?');
select colpivot('_output', $$
select r.user_id, q.body q, r.body a from responses r
join questions q on q.id = r.question_id
$$, array['user_id'], array['q'], '#.a', null);
select * from _output;
rollback;
This outputs:
user_id | 'Do you like apples?' | 'Do you like carrots?' | 'Do you like oranges?'
---------+-----------------------+------------------------+------------------------
1 | Yes | No | Yes
2 | Yes | No | No

Answer 3

You can solve this example with the crosstab function in this way
drop table if exists responses;
create table responses (
user_id integer,
question_id integer,
body text
);
drop table if exists questions;
create table questions (
id integer,
body text
);
insert into responses values (1,1,'Yes'), (2,1,'Yes'), (1,2,'Yes'), (2,2,'No'), (1,3,'No'), (2,3,'No');
insert into questions values (1, 'Do you like apples?'), (2, 'Do you like oranges?'), (3, 'Do you like carrots?');
select * from crosstab('select responses.user_id, questions.body, responses.body from responses, questions where questions.id = responses.question_id order by user_id') as ct(userid integer, "Do you like apples?" text, "Do you like oranges?" text, "Do you like carrots?" text);
First, you must install tablefunc extension. Since 9.1 version you can do it using create extension:
CREATE EXTENSION tablefunc;

Answer 4

I wrote a function to generate the dynamic query.
It generates the sql for the crosstab and creates a view (drops it first if it exists).
You can than select from the view to get your results.
Here is the function:
CREATE OR REPLACE FUNCTION public.c_crosstab (
eavsql_inarg varchar,
resview varchar,
rowid varchar,
colid varchar,
val varchar,
agr varchar
)
RETURNS void AS
$body$
DECLARE
casesql varchar;
dynsql varchar;
r record;
BEGIN
dynsql='';
for r in
select * from pg_views where lower(viewname) = lower(resview)
loop
execute 'DROP VIEW ' || resview;
end loop;
casesql='SELECT DISTINCT ' || colid || ' AS v from (' || eavsql_inarg || ') eav ORDER BY ' || colid;
FOR r IN EXECUTE casesql Loop
dynsql = dynsql || ', ' || agr || '(CASE WHEN ' || colid || '=''' || r.v || ''' THEN ' || val || ' ELSE NULL END) AS ' || agr || '_' || r.v;
END LOOP;
dynsql = 'CREATE VIEW ' || resview || ' AS SELECT ' || rowid || dynsql || ' from (' || eavsql_inarg || ') eav GROUP BY ' || rowid;
RAISE NOTICE 'dynsql %1', dynsql;
EXECUTE dynsql;
END
$body$
LANGUAGE 'plpgsql'
VOLATILE
CALLED ON NULL INPUT
SECURITY INVOKER
COST 100;
And here is how I use it:
SELECT c_crosstab('query_txt', 'view_name', 'entity_column_name', 'attribute_column_name', 'value_column_name', 'first');
Example:
Fist you run:
SELECT c_crosstab('Select * from table', 'ct_view', 'usr_id', 'question_id', 'response_value', 'first');
Than:
Select * from ct_view;

Answer 5

There is an example of this in contrib/tablefunc/.