One of the most common techniques in GIS in order to treat non-spatial data is that of the join. We use a geographical layer having geometries, as support for another layer having no location data. To this end, we will use a common field to both tables, that allows “the joining” the records from the geographic table to those of the non- geographic table. Therefore we created a new virtual table where the attributes of the non- geographic table can be used to be mapped with the geometry of the first table. For this join to work it is necessary that for every registration of geographical table corresponds a record in the non- geographic table, and only one. Because if more than one is found, the join cannot work. In that case we have a “relationship” between the tables, never a “juncture”.
For a long time, GIS software has, only, allowed to work with joins. The relationships do not stand most spatial operators applicable to joins. Continue reading “1-n joins in QGis: virtual layers”