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I am using QGIS 3.10.0 A Coruna. I would like to label a building layer with with its building number. However, sometimes I have 2-3 buildings with the same number, and I would like to override the labeling by typing in a SQL-Command asking it to make sure that every value is only used once, and that the label is positioned in the middle of the buildings.

I cannot sum up the building into a multipart, because they have, apart from the building number, different attributes I need to keep. Also, I thought about adding a column, where I use the value just once. The problem is that there are often changes within the project, and with an SQL-Command changes could be done automatically.

Is there a IF CASE WHEN THEN that can solve this problem?

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  • is the building number unique within your layer or is it only unique with a combination to an other field (e.g. street)? – eurojam Jan 16 '20 at 20:50
  • No, there is no combination with other fields. The number of the building ist different than a usual "address", they have a specific classification within our project. – rtaani Jan 17 '20 at 13:09
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I will describe 2 possibilities, the first is more easy and elegant but works only from QGIS 3.10. The second also works for older versions.

Possibility 1: Use the geometry generator for labels. It is new since version 3.10 but very powerful. The expression will be

convex_hull(collect ($geometry, "id"))

this will build a convex hull around buildings with the same ID and will be used for labeling.

enter image description here

Possibility 2: You can use Virtual Layers to achieve the same like above. A Virtual Layer will reflect all changes in the original layer. The only thing is, if you have a lot of objects, virtual layers will slow down your project. so let's have a look to the statement to get the buildings with the same ID together:

select st_convexhull(st_union(geometry)), id from buildings group by id

this will build a convex hull around buildings with the same ID. Then we can use the new virtual layer only for labeling:

enter image description here

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  • Wow! Thanks, that worked perfectly!! – rtaani Jan 17 '20 at 16:38
  • Every time, I find some awesome new ways to work with QGIS. Just... mind-blown. – Saijin_Naib Jan 17 '20 at 17:31

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