Get unique values over multiple fields

Question

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?

Example:
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

Answer 1

Your test data (layer name: Polygon):

If a Virtual Layer is acceptable, the syntax would be:

SELECT Distinct(Field_1) AS Field FROM Polygon
UNION
SELECT Distinct(Field_2) AS Field FROM Polygon
UNION
SELECT Distinct(Field_3) AS Field FROM Polygon
ORDER BY Field DESC

Then output would be: