How to create a straight line from start and end vertices of a line using Qgis?

Question

I want to create a straight line from two vertices of a line which are not straight, but I should be getting a straight line joining the start and end vertices of this curve line.

I want to attain the green line

Answer 1

There are several options to do this:

1. Create a new line-layer, toggle editing, activate snapping and draw a line by snapping to start- and endpoint.

2. Use this expression with Geometry generator or Geometry by expression (see here how): make_line (start_point ($geometry),end_point ($geometry))

3. Create a virtual layer with this query:

   select make_line (end_point (l.geometry), start_point (l.geometry))
   from line as l