[Qgis-user] Functional dependency seem not working in group

[Hans Skov-Petersen]

Dear Michael

Functional dependencies are, according the Net, applied in PostgreSQL: https://stackoverflow.com/questions/66065987/postgres-sql-column-must-appear-in-the-group-by-clause-or-be-used-in-an-aggrega

And accordingly, you shouldn't worry about it in PostGIS. The funny thing is that sometime it works. Sometimes it doesn't.

Cheers
Hans

-----Original Message-----
From: Qgis-user <qgis-user-bounces at lists.osgeo.org> On Behalf Of Reetz, Michael (NLPV) via Qgis-user
Sent: 17. oktober 2022 08:47
To: qgis-user at lists.osgeo.org
Subject: Re: [Qgis-user] Functional dependency seem not working in group

Hello Hans,

as far as I know, it is an SQL rule, that all selected attributes that are not queried with an aggregate function have to be part of the group by clause. If you run that query, your database server will check the syntax and throw that error. I'm not sure if the SQL server realizes the 'functional dependency' in your query as assumed by you.

Cheers,
Michael

-----Ursprüngliche Nachricht-----
Von: Qgis-user <qgis-user-bounces at lists.osgeo.org> Im Auftrag von Hans Skov-Petersen via Qgis-user
Gesendet: Montag, 17. Oktober 2022 08:00
An: Qgis-user at lists.osgeo.org
Betreff: [Qgis-user] Functional dependency seem not working in group


Dear Friends

I have, what appears to be a classic problem: When running a selection including a group statement I get the good old 'ERROR:  column "urban.geom" must appear in the GROUP BY clause or be used in an aggregate function'.

I thought 'functional dependency' was in place to take care of that. The way I understand it, is that functional dependency will, as applied in e.g. the group statement, realize that e.g. the geom field is uniquely related to the id field (which is referred in the group statement). Am I right in my assumption?

This is the ode causing the trouble:

drop table if exists stevns_test_urban_ped; create table stevns_test_urban_ped as
  select urban.id as id, urban.geom as geom, min(urban.pop2019) as pop2019, count(forest) as ped, min(urban.pop2019)/(count(forest)+1)::float as pop_forest_ped
  from (select * from stevns_test_p where lu_type = 1) as urban
  join (select * from stevns_test_p where lu_type = 2) as forest on ST_Distance(urban.geom, forest.geom) < 1000 and forest.geom && (ST_Expand(urban.geom, 1000))
  group by urban.id
;

Never mind the logics or meaning :-)

Thanks
Cheers
Hans

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