Linear referencing using virtual layers in QGIS

I am trying to replicate the behaviour explained on this page.

There are two initial layers :

• One network layer (line geometry), called base_lines (fields : LB_idRiver)
• One event layer (non-geometrical table), called linear_events and containing information about the beginning (eFrom) and the end of the event (eTo) (fields : fid, LE_from, LE_value)

From there, I need to, using virtual layers in QGIS :

• Create two point layers showing the position of the beginning and the end of the events along the base_line layer
• Create a new line layer extracting the parts of the base_line layer between the points.

So far, my query for the first task looks like this :

SELECT
    e.fid,
    e.'LE_FROM' AS Debut, 
    e.'LE_idRiver' AS RouteID, 
    e.'LE_Value' AS val,
    r.'LB_idRiver',
    ST_Line_Interpolate_Point(r.geometry, (e.'LE_FROM' - ST_M(ST_StartPoint(r.geometry))) / (ST_M(ST_EndPoint(r.geometry)) - ST_M(ST_StartPoint(r.geometry)))) as geom
FROM "Linear_events" as e
Join "base_lines" as r ON (e.'LE_idRiver' = r.'fid')

The table is created, but the geometry creation fails (NULL gets returned for each part of the geom alias).

What changes have to be made to the query to make it work as intended?
Once this query works, how is it possible to get the second one working?

(On a side-note, I’d be very interested in advice concerning virtual layer creation, as the interface is not really comfortable in QGIS)

---

I finally made it work based on the advice here :

For the first query :

SELECT
    e.fid AS Event_ID/*:int*/,
    e.'LE_FROM' AS Debut /*:real*/, 
    e.'LE_idRiver' AS RouteID /*:int*/, 
    e.'LE_Value' AS val /*:int*/,
    r.'LB_idRiver',
    ST_Line_Interpolate_Point(r.geometry, e.'LE_FROM' /st_length(r.geometry)) as geom /*:point:2056*/
FROM  "Linear_events"as e
Join    "base_lines" as r ON (e.'LE_idRiver' = r.'fid')

And the second query :

SELECT
    e.fid /*:int*/,
    e.'LE_idRiver' AS RouteID /*:int*/, 
    r.'LB_idRiver' AS RivName /*:text*/,
    e.'LE_FROM' AS Debut /*:real*/, 
    e.'LE_TO' AS Fin /*:real*/, 
    e.'LE_Value' AS val /*:int*/,
    ST_Line_Substring(r.geometry, e.'LE_FROM' / st_length(r.geometry), e.'LE_TO' / st_length(r.geometry)) as geom /*:linestring:2056*/
FROM  "Linear_events"as e
Join    "base_lines" as r ON (e.'LE_idRiver' = r.'fid') 

SELECT id /*:int*/, ST_Line_Interpolate_Point(geometry, 0.3) AS geom /*:point:4326*/ FROM demo_pts