Distance betweeen classified Points

I got two sets of points.

1. The first set has a unique ID
2. The second set references to this ID (one-to-many).

So I merged the two sets. Now every point in the joined table has the X and Y coordinates from the related first set.

Name;ID;X;Y;X_2;Y_2;Distance

Every X and Y is unique but X_2 and Y_2 repeats from a given set.

Now I would like to calculate the distance between (X,Y) and (X_2,Y_2) in the new column for each row.

I tried distance() but I'm way too new in Python to get it working.

I also tried some tools like Distance Matrix, but I got results for all distances (or to the number defined) but not only to the distances in one row. Or between Points sharing the same ID.

Executed in QGIS 2.18 and QGIS 3.0.3

In the Field Calculator/Expression dialog, please try

distance(make_point("X", "Y"), make_point("X_2", "Y_2"))

Function distance() returns the minimum distance (based on spatial ref) between two geometries in projected units.

Syntax: distance(geometry a, geometry b) 