How to eliminate repeated field with GROUP BY clause? - postgresql

Question

I have 3 tables called:
1.app_tenant pk:id, fk:pasar_id
---+--------+-----------+
id | nama | pasar_id |
----+--------+-----------+
1 | joe | 1 |
2 | adi | 2 |
3 | adam | 3 |
2.app_pasar pk:id
----+------------- +
id | nama |
----+------------- +
1 | kosambi |
2 | gede bage |
3 | pasar minggu |
3.app_kios pk:id, fk:tenant_id
----+---------------+----------
id | nama |tenant_id
----+-------------- +----------
1 | kios1 |1
2 | kios2 |2
3 | kios3 |3
4 | kios4 |1
5 | kios5 |1
6 | kios6 |2
7 | kios7 |2
8 | kios8 |3
9 | kios9 |3
Then with a LEFT JOIN query and grouping by id in every table I want to displaying data like this:
----+---------------+------------+-----------
id | nama_tenant |nama_pasar |nama_kios
----+-------------- +------------------------
1 | joe |kosambi |kios 1
2 | adi |gede bage |kios 2
2 | adam |pasar minggu|kios 3
but after I execute this query, data are not shown as expected. The problem is
redundancy in the nama_tenant field. How can I eliminate repeated nama_tenantrecords?
This is my query:
select a.id,a.nama as nama_tenant,
b.nama as nama_pasar,
c.nama as nama_kios
from app_tenant a
left join app_pasar b on a.id=b.id
left join app_kios c on a.id= c.tenant_id
group by
a.id,
b.id,
c.id
Table definitions:
CREATE TABLE app_tenant (
id serial PRIMARY KEY,
nama character varying,
pasar_id integer);
CREATE TABLE app_kios (
id serial PRIMARY KEY,
nama character varying,
tenant_id integer REFERENCES app_tenant);

Answer 1

The problem is that tenants can have multiple kiosks. From your sample data it looks like you want to display the first kiosk of every tenant (although "first" is a vague concept on strings, here I use alphabetical sort order). Your query would be like this:
SELECT t.id, t.nama AS nama_tenant, p.nama AS nama_pasar, k.nama AS nama_kios
FROM app_tenant t
LEFT JOIN app_pasar p ON p.id = t.pasar_id
LEFT JOIN (
SELECT tenant_id, nama, rank() OVER (PARTITION BY tenant_id ORDER BY nama) AS rnk
FROM app_kios
WHERE rnk = 1) k ON k.tenant_id = t.id
ORDER BY t.id
The sub-query on app_kios uses a window function to get the first kiosk name after sorting the names of the kiosk for each tenant.
I would also suggest to use meaningful aliases for table names instead of simply a, b, c.