[Qgis-developer] Aggregates within expression engine

Neumann, Andreas a.neumann at carto.net
Wed Mar 16 00:53:51 PDT 2016

Hi Paolo, 

And what would be the alternative syntax that is easier than SQL syntax?

Please try to be more specific rather than just saying SQL syntax is
complicated. Syntax that allows to use aggregate functions and grouping
for the following use cases: 

Aggregate functions: 

All data types: 

 	* Count
 	* Count distinct
 	* Count missing values (null)

Text data types: 

 	* string_agg/join: with delimiter and sort option

Numeric data types: 

 	* Sum
 	* Mean
 	* Median
 	* Minimum
 	* Maximum
 	* Range
 	* St dev(pop/sample)
 	* Minority
 	* Majority
 	* Variety
 	* Q1
 	* Q3
 	* IQR

Date/Time types: 

 	* Minimum date/time
 	* Maximum date/time
 	* Range between minimum/maximum date/time


Grouping options: 

 	* All records in the attribute table
 	* A given grouping field or expression
 	* Currently Selected features
 	* Selection based on table relations (e.g. aggregation of all owners
of a parcel)
 	* Optional: have the same function grouped by each symbol category or



On 2016-03-16 08:36, Paolo Cavallini wrote:

> Il 16/03/2016 08:30, Neumann, Andreas ha scritto:
>> Why do you think SQL aggregate syntax (e.g. as outlined
>> at http://www.postgresql.org/docs/9.5/interactive/functions-aggregate.html)
>> is a no-go? Can you explain this in detail? QGIS is much closer to a
>> database then it is to a spreadsheet - in fact for most serious QGIS
>> work you store your data in a SQL database.
>> It was my impression that QGIS tries to maintain expression syntax
>> compatible with SQL wherever possible. I think it would be a good thing.
> Hi Andreas,
> based on my lengthy experience with courses, I'm pretty sure SQL
> commands are out of reach for most "normal" GIS users.
> It is fine to have access to SQL for power users, but aggregate
> functions should IMHO be available to anyone.
> All the best.

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