For creating a legend of a map, I need to get a list of unique values over multiple attribute columns of a layer. I know how to get a list of unique values within one column - is it possible to get a list of unique values over multiple columns?
Polygon 1: Field 1: x, Field 2: a, Field 3: b
Polygon 2: Field 1: a, Field 2: c, Field 3: z
Polygon 3: Field 1: x, Field 2: b, Field 3: z
Now I need a list with the values that are unique over the fields 1, 2 and 3. For the example that would be x, a, b, c, z.
I am using QGIS Version 2.14.1