I am working with QGIS 2:14, for now, I have a qgs project with a lot of layers and I want to create a label in the print-composer with the result of a calculated expression from one of this layers.
In one of my layers (recycling_points.shp ) I have 150 records with the recycling points in the city. Then, for every record I have a field (num_containers) with the number of containers of every recycling point, and I want to create a text label in the print-composer, based on a calculated expression, with the number of containers in my city, but I don't know how can do it correctly.
I've tried with the next expression:
but is invalid.