postgres one to many flattened return - postgresql

Question

I am trying to write a postgres query that uses 3 tables: people, attribute, and a people_attribute join
people table:
id, name
attribute table:
id, name, attr_group
people_attribute join:
people_id, attribute_id
desired output:
name | fav_colors | fav_music | fav_foods
-----------------------------------------------------------------
michael | red,blue,green | pop,hip-hop,jazz | pizza,burgers,tacos
bob | orange,green | null | tacos,steak,fish
...etc
The tags can vary from none to ~12 for each attr_group
Here is the query I am working with:
select
p.id,
p.name,
(case when a.attr_group like 'fav_colors' then string_agg(a.name, ',') else null end) as fav_colors,
(case when a.attr_group like 'fav_music' then string_agg(a.name, ',') else null end) as fav_music,
(case when a.attr_group like 'fav_foods' then string_agg(a.name, ',') else null end) as fav_foods,
from people as p
join people_attribute as pa on pa.people_id = p.id
join "attribute" as a on a.id = pa.attribute_id
group by 1,2,a.attr_group
order by 1 asc;
which returns:
name | fav_colors | fav_music | fav_foods
-----------------------------------------------------------------
michael | red,blue,green | null | null
michael | null | pop,hip-hop,jazz | null
michael | null | null | pizza,burgers,tacos
bob | null | null | null
bob | orange,green | null | null
bob | null | null | tacos,steak,fish
I feel like I'm getting close, but am unsure how to flatten this out to achieve the desired output as shown above. Any help would be greatly appreciated!

Answer 1

You want to use filter for this:
select p.id,
p.name,
string_agg(a.name, ',') filter (where a.attr_group = 'fav_color') as fav_colors,
string_agg(a.name, ',') filter (where a.attr_group = 'fav_music') as fav_music,
string_agg(a.name, ',') filter (where a.attr_group = 'fav_foods') as fav_foods,
from people as p
join people_attribute as pa
on pa.people_id = p.id
join "attribute" as a
on a.id = pa.attribute_id
group by p.id, p.name
order by 1 asc;
Using filter passes only values that match the filter where condition into the aggregation.
The reason yours was showing three rows per people record is because you added attribute.attr_group to your group by. You had no choice since you were using attribute.attr_group in your case conditionals.
Using filter makes attribute.attr_group part of the aggregation, so you do not have to include it in your group by list.