Join three rows if the same value in one column - sql

Question

There is a Postgres database and the table has three columns. The data structure is in external system so I can not modify it.
Every object is represented by three rows (identified by column element_id - rows with the same value in this column represents the same object), for example:
key value element_id
-----------------------------------
status active 1
name exampleNameAAA 1
city exampleCityAAA 1
status inactive 2
name exampleNameBBB 2
city exampleCityBBB 2
status inactive 3
name exampleNameCCC 3
city exampleCityCCC 3
I want to get all values describing every objects (name, status and city).
For this example the output should be like:
exampleNameAAA | active | exampleCityAAA
exampleNameBBB | inactive | exampleCityBBB
exampleNameCCC | inactive | exampleCityCCC
I know how to join two rows:
select a.value as name,
b.value as status
from the_table a
join the_table b
on a.element_id = b.element_id
and b."key" = 'status'
where a."key" = 'name';
How is it possible to join three columns?

Answer 1

You can try below
DEMO
select a.value as name,
b.value as status,c.value as city
from t1 a
join t1 b
on a.element_id = b.element_id and b."keys" = 'status'
join t1 c on a.element_id = c.element_id and c."keys" = 'city'
where a."keys" = 'name';
OUTPUT
name status city
exampleNameAAA active exampleCityAAA
exampleNameBBB inactive exampleCityBBB
exampleNameCCC inactive exampleCityCCC

Answer 2

One option is to simply add another join for each value you need (this is one of the big disadvantages of the EAV (anti) pattern you are using:
select a.value as name,
b.value as status,
c.value as city
from the_table a
join the_table b on a.element_id = b.element_id and b."key" = 'status'
join the_table c on a.element_id = c.element_id and c."key" = 'city'
where a."key" = 'name';
Another option is to aggregate all key/value pairs for an element into a JSON then you can easily access each one without additional joins:
select t.element_id,
t.obj ->> 'city' as city,
t.obj ->> 'status' as status,
t.obj ->> 'name' as name
from (
select e.element_id, jsonb_object_agg("key", value) as obj
from element e
group by e.element_id
) t;
If the table is really big this might be a lot slower than the join version due to the aggregation step. If you limit the query to only some elements (e.g. by adding a where element_id = 1 or where element_id in (1,2,3)) then this should be quite fast.
It has the advantage that you always have all key/value pairs for each element_id available regardless on what you do. The inner select could be put into a view, to make things easier.
Online example: https://rextester.com/MSZOWU37182

Answer 3

Seems like you want to PIVOT
One way to do that is via conditional aggregation.
select
-- t.element_id,
max(case when t.key = 'name' then t.value end) as name,
max(case when t.key = 'status' then t.value end) as status,
max(case when t.key = 'city' then t.value end) as city
from the_table t
group by t.element_id;
db<>fiddle here
Or use crosstab:
select
-- element_id,
name,
status,
city
from crosstab (
'select t.element_id, t.key, t.value
from the_table t'
) as ct (element_id int, name varchar(30), status varchar(30), city varchar(30));
But if you do like those joins, here's a way
select
-- el.element_id,
nm.value as name,
st.value as status,
ci.value as city
from
(
select distinct t.element_id
from the_table t
where t.key in ('name','status','city')
) as el
left join the_table as nm on (nm.element_id = el.element_id and nm.key = 'name')
left join the_table as st on (st.element_id = el.element_id and st.key = 'status')
left join the_table as ci on (ci.element_id = el.element_id and ci.key = 'city');