Concatenating Attributes from Differing Features (Grouped By Value) Into Single Label

Question

I have been using QGIS 3.28.0 and want to be able to concatenate the attribute values from differing features into the same label with each concatenated value on a separate line. I would group the aggregate calculation by one of the attributes. My proposed attribute table would look something like the following:

The aim would be to have a single label for the 3 features (these have the same coordinates) like below:

1
2sometext3sometext
6sometext9sometext
5sometext7sometext

(The concatenation will be grouped by Column A)

To achieve this I tried using the following expression for the label value:

"Column A" ||  '\n' || concatenate ( to_string("Column B") || 'some text' ||  to_string("Column C") || 'some text' ||  '\n' , " Column A")

The expression nearly worked but has given me a trailing blank line. Does anyone have any suggestions for getting rid of this blank line or can anyone offer a better expression to achieve my aims. Any additional advice on how to further sort this label by the numerical values in Column B would also be great.

Answer 1

Use the following expression. The basis for the solution is the array_agg() function (line 7), where you concatenate the attribute fields and the texts you want (first argument). Add a second argument group_by and group by Column A. That's it, basically.

The rest of the expression is to create the correct formatting, see below for explanations:

case 
when array_agg ($id, group_by:="Column A")[0] = $id
then 
replace (
    "Column A"  ||  '\n'   ||
    array_to_string (
        array_agg ("Column B"  || ' sometext '  || "Column C"  || ' sometext'  , group_by:="Column A")
    ), 
    ',', 
    '\n' 
)
end

---

Apart from line 7 of the above expression, the rest is to create the correct format:

• Create only one label per group of common values in Column A using a case ... when condition with the expression on line 2
• Convert the array to a string (line 6)
• add the value of Column A followed by a linebreak at the beginning (line 2)
• Replace the commas (originating from the array) to linebreaks (lines 4 and 9-10)