Connecting points with line in a specific sequence

Question

I have 6 points in one layer as shown in the first image. I am trying to:

connect points with lines in a required sequence.
Save data of line connections (From_point and To_point) in the seperate layer.

For demo, I have manually connected the lines (green) and shown the sequence (red) with numbers in Image 1. The sequence of connecting lines should be as follows:

Sequence
From_point
To_point

1 (building1) to (Junction)
7b9f
aab4

2 (building2) to (Junction)
d052
aab4

3 (Junction) to (Breaker)
aab4
edb3

4 (Breaker) to (Station)
edb3
c037

5 (Sation)) to (MainSource)
c037
4d3c

Initial attempt with points to path:
I tried to achieve this Menu Processing / Toolbox / points to path and Points2One plugin but it does not maintain the required sequence of connections as shown in Image 2

lejedi76 · Accepted Answer

All in one solution with virtual layer

For your probleme you can indeed use virtual layer, I didn't know you want use this method :

with
build_segment as (
select 
  a.mrid as start_point, 
  b.mrid as end_point,
  makeline(a.geometry, b.geometry) as geometry
from <layername> a
left join <layername> b on (a.id + 1 = b.id  ))

select
  * 
from build_segment
where geometry is not null

Correct answer by lejedi76 on June 4, 2021

lejedi76 · Answer

Solution with algorithms and expressions
For the fist part of your problem :
You could use the geometry by expression algorithm and use this expression :
collect_geometries(
    array_foreach(
        aggregate( 
            'LAYERNAME' ,
            'array_agg', 
            "mrid", 
            filter:="id" =  attribute(@parent, 'id')+1),
        make_line($geometry, geometry(get_feature('LAYERNAME', 'mrid',@element)))
    )
)

It's not the best approach to what mentioned Kadir. If there are 2 ore more children this expression will build a multilinestring.
For the second part,
apply the multiparttosinglepart algorithm on the output of the first treatment. The mrid field correponds to the start and you can obtain the end by adding a new fied on the ouput with this formula :
aggregate('LAYERNAME',
'concatenate', "mrid", filter:= intersects($geometry,end_point(geometry(@parent))))

*intersects is used in place of equals
** LAYERNAME is the SOURCE LAYER (point layer)

Connecting points with line in a specific sequence

2 Answers

All in one solution with virtual layer

Solution with algorithms and expressions

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Sequence	From_point	To_point
1 (building1) to (Junction)	7b9f	aab4
2 (building2) to (Junction)	d052	aab4
3 (Junction) to (Breaker)	aab4	edb3
4 (Breaker) to (Station)	edb3	c037
5 (Sation)) to (MainSource)	c037	4d3c