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Using QGIS, I have to map the general area of my project. However, this area depends on another layer representing sub-areas.

I know that I can use the dissolve tool on the second layer to get the extent, which gives me the exact result I am looking for.

However, as the second layer will undergo further modifications which might impact its extent, I was wondering if there was a way to create some kind of a "virtual layer" that represents the extent of the sub areas from the second layer? I wish to avoid dissolving the layers over and over again.

I can't use a simple style trick, as my main extent has to be surrounded by a dash line (management requirement).

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You can create a virtual layer with the dissolved geometry. Go to:
Layer -> Create Layer -> New Virtual Layer ...

In the Query section you write

SELECT st_union(geometry)
FROM "yourlayername"

and then press Add.
The function st_union dissolves the geometry of the layer. Yourlayername stands for the name of your layer representing the sub-areas.

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