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For example, I have two layers:

  • A polygon layer city block with a ID field city block index.
  • A point layer parking lot with a field parking lot size.

I want to know the total parking lot size for each feature in the land use layer, so I use the spatial join function. Here's my workflow:

  1. Use join attributes by location (summary) to create a new temporary joined layer who contains a field called parking lot size_sum.
  2. Go to the property panel of the city block layer and join the parking lot size_sum field as a new field called joined layer_parking lot size_sum.
  3. In the city block layer, create a new field called total parking lot size .
  4. Copy values from joined layer_parking lot size_sum and paste them into total parking lot size.
  5. Delete the temporary joined layer.

All I want to do is to add a new field into the original city block layer by summarizing the parking lot size field. But I have to go through all the steps above. Is there a simpler way to do that?

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

With the following Query, it is possible to know the total 'parking lot size' for each 'city block'.

SELECT cb.*, SUM(pl."parking lot size") AS pl_sum
FROM "city block" AS cb
JOIN "parking lot" AS pl ON st_contains(cb.geometry, pl.geometry)
GROUP BY cb."city block index"
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  • Thanks. I can't quite differentiate whether it's the layer name or the field name each name in your answer refers to. So I differentiate them a little bit in my question. Could you please also update your answer accordingly? Aug 25 '20 at 9:34
  • Please check updates
    – Taras
    Aug 25 '20 at 11:39
  • Thanks! It helps. Though, I guess you made a typo by typing pl as pls in the first line. Also, it seems that a virtual layer has a performance issue compared to a regular layer. It takes much more longer time to respond for each of my operations, like panning or opening the attribute table. Aug 27 '20 at 9:34
  • 1
    you are right. And yes the performance of VLs is not their best quality.
    – Taras
    Aug 27 '20 at 15:18

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