QGIS – Selecting Features by Expression

select-by-expression

I have a set of features in a single layer that have been created during various periods of time.

In a layer attribute table, there is a column kreirao_datum which means created on a date

I would like to select all features created within a single calendar year, for example 2021, using 'Select by Expression', but I can not figure out how to write the expression.

It should probably be something like "kreirao_datum" = '2021-07-29T09:45:01.000', and it works for a single matching record.

But how can I write expression that would select all records created within a 2021 year, "kreirao_datum" = '2021_????'?

I am very well aware that I can use Filter within attribute table, and it also works, but I am trying to determine what would be the right expression for 'Select by Expression'.

Best Answer

Use function year() to extract the year form your date:

year ("kreirao_datum") = 2021

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The expression works correctly.
The problem is related to the circle variable which creates a circle for each point.
When we use within (@selected_pt, @circle), QGIS selects all the circles that contain the selected point. Below a screenshot that demonstrates the above, the selected point is contained only in 5 circles.

QGIS Feature Selection – Selecting Features Visible in Map Extent Using QGIS

The trick is to create a function that access QGIS graphical interface, and that is piped to the query of the select by expression.

Open a function editor (from anywhere, including from field calculator) and create a new function that reads the canvas extent and returns it as a geometry.

from qgis.core import *
from qgis.gui import *
from qgis.utils import iface

@qgsfunction(args='auto', group='Custom')
def currentExtent(feature, parent):
    return QgsGeometry.fromRect(iface.mapCanvas().extent())

Open the select by expression and use intersects($geometry,currentExtent()) (or within(..) for entirely contained polygons)

The same function can be used in a virtual layer as shown here, that you could eventually modify to compute the buffer.

Best Answer

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