转置sql结果，使一列进入多列

Question

I'm trying to get data out of a table for a survey in a particular format. However all my attempts seems to hand the DB because of too many joins/too heavy on the DB.

My data looks like this:

id, user, question_id, answer_id, 
1,   1,   1,           1
3,   1,   3,           15
4,   2,   1,           2
5,   2,   2,           12
6,   2,   3,           20

There are roughly 250,000 rows and each user has about 30 rows. I want the result to look like:

user0, q1, q2,   q3 
1,     1,  NULL, 15
2,     2,  12,   20 

So that each user has one row in the result, each with a separate column for each answer.

I'm using Postgres but answers in any SQL language would be appreciated as I could translate to Postgres.

EDIT: I also need to be able to deal with users not answering questions, ie in the example above q2 for user 1.

Answer 1

Consider the following demo:

CREATE TEMP TABLE qa (id int, usr int, question_id int, answer_id int);
INSERT INTO qa VALUES
 (1,1,1,1)
,(2,1,2,9)
,(3,1,3,15)
,(4,2,1,2)
,(5,2,2,12)
,(6,2,3,20);

SELECT *
FROM   crosstab('
    SELECT usr::text
          ,question_id
          ,answer_id
    FROM qa
    ORDER BY 1,2')
 AS ct (
     usr text
    ,q1 int
    ,q2 int
    ,q3 int);

Result:

 usr | q1 | q2 | q3
-----+----+----+----
 1   |  1 |  9 | 15
 2   |  2 | 12 | 20
(2 rows)

user is a reserved word . Don't use it as column name! I renamed it to usr .

You need to install the additional module tablefunc which provides the function crosstab() . Note that this operation is strictly per database . In PostgreSQL 9.1 you can simply:

CREATE EXTENSION tablefunc;

For older version you would execute a shell-script supplied in your contrib directory. In Debian, for PostgreSQL 8.4 , that would be:

psql mydb -f /usr/share/postgresql/8.4/contrib/tablefunc.sql

Answer 2

Erwins answer is good, until missing answer for a user shows up. I'm going to make an assumption on you....you have a users table that has one row per user and you have a questions table that has one row per questions.

select usr, question_id
from users u inner join questions q on 1=1
order by 1,

This statement will create a row for every user/question, and be in the same order. Turn it into a subquery and left join it to your data...

select usr,question_id,qa.answer_id
from
(select usr, question_id
from users u inner join questions q on 1=1
)a
left join qa on qa.usr = a.usr and qa.question_id = a.usr
order by 1,2

Plug that into Erwins crosstab statement and give him credit for the answer :P

Answer 3

I implemented a truly dynamic function to handle this problem without having to hard code any specific number of questions or use external modules/extensions. It also much simpler to use than crosstab() .

You can find it here: https://github.com/jumpstarter-io/colpivot

Example that solves this particular problem:

begin;

create temp table qa (id int, usr int, question_id int, answer_id int);
insert into qa values
 (1,1,1,1)
,(2,1,2,9)
,(3,1,3,15)
,(4,2,1,2)
,(5,2,2,12)
,(6,2,3,20);

select colpivot('_output', $$
    select usr, ('q' || question_id::text) question_id, answer_id from qa
$$, array['usr'], array['question_id'], '#.answer_id', null);

select * from _output;

rollback;

Result:

 usr | 'q1' | 'q2' | 'q3' 
-----+------+------+------
   1 |    1 |    9 |   15
   2 |    2 |   12 |   20
(2 rows)