Summing area of by several attributes in QGIS

Question

I have a shapefile that contains building footprints with multiple addresses with different functions (with each an usable area). I would like to calculate the sum of the usable area, per building "id", per same "function", as such:

id	function	area
1	wo	10
1	wi	2
2	wo	50
1	wo	10

↓ calculate sum (area) when same id and same function

id	function	area
1	wo	20
1	wi	2
2	wo	50

I tried to use the following expression:

sum(expression [,group_by] [,filter])

But the group_by() is only possible once (for instance to group the "id").

How can we calculate the sum of the two features only when these have the same "id" and same function?

Answer 1

Use a combination of the two fields like concat: "id" || "function" for the group_by() part of the expression:

sum("area", "id" || "function")

Answer 2

There is also a possibility using a "Virtual Layer" through Layer > Add Layer > Add/Edit Virtual Layer....

Let's assume there is a point layer called "test" with its attribute table, see image below.

With the following query, it is possible to calculate the sum of the features only when these have the same "id" and the same "function", as well as getting rid of duplicates.

SELECT
    SUM(value) AS sumvalue,
    COUNT(*) AS numgrouped,
    "id" || "function" AS grouped,
    geometry AS geom
FROM
    "test"
GROUP BY
    "id", "function"

The output point layer with a new attribute table will look like

Last but not the least, keep in mind that:

• the number of features in the final output will be less (rarely the same) than in the original shapefile
• grouping values of features with neglecting their geometries is not the same as the grouping values of features with preserving their geometries