Postgresql - conditional statement to refer to other columns - sql

Question

I currently have a dataset that looks like this:
Personid | Question | Response
1 | Name | Daniel
1 | Gender | Male
1 | Address | New York, NY
2 | Name | Susan
2 | Gender | Female
2 | Address | Boston, MA
3 | Name | Leonard
3 | Gender | Male
3 | Address | New York, NY
I also have another table that looks like this (just the person id):
Personid
1
1
1
2
2
2
3
3
3
I want to write a query to return something like this:
Personid | Name | Gender | Address
1 |Daniel | Male | New York, NY
2 | Susan | Female | Boston, MA
3 |Leonard | Male | New York, NY
I think it's a mix of some sort of "transpose" (not sure if it's even available in SQL) and conditional statement on just the gender, but I'm having issues with getting the end result. Could anyone offer any advice?

Answer 1

Easiest way is just to link to the question table three times with different aliases.
select
p.person_id,
n.response as name,
g.response as gender,
a.response as address
from
person p
join question n
on n.personid = p.personid and n.question = 'Name'
join question g
on g.personid = p.personid and g.question = 'Gender'
join question a
on a.personid = p.personid and a.question = 'Address'
I'm assuming that your person table only has 3 rows not the 9 you've listed. if there are really 9, then just do a select distinct.

Answer 2

This is a textbook example of a pivot table. In postgresql it is implemented by the CROSSTAB function, which is available from the TABLEFUNC additional extension module.
If your need is really as simple as the provided MCVE, multiple JOIN’s might be enough, but in more complicated situations CROSSTAB is really the way to go, and worth the pain of installing an additional module, if it is not installed by default by your distro. In short, if your initial table is called dataset, and personid is an INT:
-- To execute as superuser. Be sure you have installed the extension
-- package. Execute once to install, it will stay in your database
-- ever since.
CREATE EXTENSION TABLEFUNC;
-- As normal user
SELECT * FROM CROSSTAB($$
SELECT personid, question, response FROM dataset
$$) AS ct(person INT, name TEXT, gender TEXT, address TEXT);
person | name | gender | address
--------+----------+---------+---------------
1 | Daniel | Male | New York, NY
2 | Susan | Female | Boston, MA
3 | Leonard | Male | New York, NY
(3 rows)
You can add WHERE clauses, JOIN with other tables, etc., according to your needs.