QGIS3: Neat trick to convert color name into color rgb string

color_mix_rgb('blue', 'white', 0)

Arguments to the function are color1: a color string, color2: a color string, ratio: the ratio at which the two colors will be mixed. The output is:

'0,0,255,255'

The usecase is converting a color name, stored in a field (field name is "color_code"), to a color string to be used for symbolizing layer features, like so:

color_mix_rgb("color_code", 'white', 0.2)

which will give me a bit of a pastel tone, by mixing white into the given "color_code" (blue, red, green, etc.) with a ratio of 0.2

QGIS3: Find Minimum/Maximum/Mean Raster Value (Elevation) for Polygon Overlay

For finding raster value statistics (Min, Max, Mean) for a polygon overlay, I apply this expression in a virtual field by using the field calculator. The raster layer that I'm querying is named

'DGM_merged'

The functions is looking for the min. raster value at the polygon's nodes

array_min(
	array_foreach(
		generate_series(1, num_points($geometry), 1),
		raster_value('DGM_merged', 1, 
			point_n(nodes_to_points($geometry), @element)
		)
	)
)

QGIS3: Virtual Layers & Spatialite/SQLite/SQL Queries - Intersecting Lines With Polygons

SQL and Spatialite make super effective table and geometry operations possible. With the implemantation of Virtual Layers, QGIS has now a built in functionality to run such queries very easily and without the need for preparation of databases. The below SQL snippet, i.e., can be used to intersect features of a line and a polygon layer, and calculate the summed segment lengths for each single polygon. The usecase here was, that I needed to know the cummulated lengths of trail crossing single parcels.

SELECT 
  t.Name AS Trail_Name,
  g.GNR AS Parcel_NR,
  SUM(LENGTH(INTERSECTION(t.geometry, g.geometry))) AS SUM_L_inP,
  COLLECT(INTERSECTION(t.geometry, g.geometry)) as geom
FROM Trails AS t JOIN Parcels AS g ON INTERSECTS(t.geometry, g.geometry)
GROUP BY g.GNR