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Given a relation in QGIS 3.x (I have a layer A of points (150 geometries) and an attribute table B of users with 500 records, and each one of this registered users is associated to a point geometry through a relation), is it possible to store in a new field of the layer A (points) the number of registered users that are associated to each point, using the established relation?

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You can summarize your table B to get a new table with one column being the common ID you use for your join and the second column being the count of each ID.

To do that you could use a virtual layer with this SQL query :

SELECT JOIN_FIELD_NAME, COUNT(JOIN_FIELD_NAME) as 'number of users' from TABLE_B Group by JOIN_FIELD

(just replace JOIN_FIELD_NAME and TABLE_B by the actual name).

This new table now has a one to one relationship with your Layer A and and after a join you could update your new field in layer A with the user count, but as a virtual layer is dynamic you should just do a join, this way if you add or remove user your count will stay up to date

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