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I have two polygon layers that partially overlap: buidings and plots. Each layer has 126 features.

I want to create a Virtual Layer that is simply an addition of the two layers, thus it should contain 254 features (all features from buildings + all features from plots).

This is what I tried:

select b.geometry, p.geometry as geom from building as b, plot as p 
select st_union (b.geometry, p.geometry) as geom from building as b, plot as p

I also tried replacing st_union by combine. However, every time I get 15'876 features, thus 126 * 126 instead of 126 + 126 features.

How should the query be to get a simple addition of the features of both layers?

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You should union the 2 selects like:

SELECT b.geometry FROM building b UNION SELECT p.geometry FROM plot p;
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    Keep in mind that if more fields should be selected, pay attention that dimension of each selection have to be equal, otherwise it may lead to this error: Query preparation error on PRAGMA table_info(_tview): SELECTs to the left and right of UNION do not have the same number of result columns Commented Sep 1, 2021 at 11:35
  • @Taras thanks for this clarification. So I guess converting geometries using centroid (to point), buffer (to polygon) or boundary/buffer-boundary (to line) will solve the problem, right? Commented Sep 1, 2021 at 13:49
  • It depends what you are trying to do. My message was more about the dimension of your selection i.e. attributes you are trying to select (geometry is "one of them") Commented Sep 2, 2021 at 6:38

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