Is it possible to join the same table twice in QGIS?

Question

Basically, I have the layers above:

As shown in the image, I'm trying to join the layer lines with the layer points two times in a row so the labels of origin_id and extremity_id appear in the attribute table of the layer lines.

I tried with Joins in QGIS but unfortunately, I can only join the layer points once.

Is there a solution, please?

Here is an example:

And here is the expected output:

Why would you want to join twice? Does not seem to make sense to me. What is your final goal you want to achieve? Joining once, you can add all attributes from one layer to the other, no neet to repete this for each attribute separately. — Babel, Jan 01 '23 at 13:51
In the layer Lines there is only ids of origin and extremity. I need to get their labels from another layer that is Points. That's why I need to join twice.. — Timeless., Jan 01 '23 at 13:54
Not clear what you join: you write about layers lines and points, but your screenshot shows a layer cables.. is this the same with the lines layer? — Babel, Jan 01 '23 at 13:55
I think there is a design mistake if you think you have to join a table twice, maybe you want to rethink your problem. It's only a guess for the moment... — swiss_knight, Jan 01 '23 at 13:57
I added an example to my question. I think that @Babel's solution (duplicating Points) will allow me to make appear two fields origin_label and extrem_label in the layer Lines. — Timeless., Jan 01 '23 at 14:03
I added the expected output as well. The goal is make a new field that concatenate the two labels. It's a very basic problem but looks like I failed to describe it very well. By the way, duplicating the layer Points worked for me. — Timeless., Jan 01 '23 at 14:12

Babel · Accepted Answer · 2023-01-01T14:19:41.707

8

What you want to do is not joining layers, but creating two new attributes. You can achieve this with field calculator using this expression on the lines layer:

attribute (
    get_feature(
        'Points',  -- name of the points layer
        'id',  -- name of the attribute on the points layer
        extreme_id -- name of the attribute on the lines layer
    ),
    'label' -- name of the attribute on the points layer
)

edited Jan 01 '23 at 14:19

answered Jan 01 '23 at 14:12

Babel

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1

I added a second clause for the origins as well and it worked. Thank you so much Babel ;) – Timeless. Jan 01 '23 at 14:16

score 3 · Answer 2 · edited Jan 02 '23 at 15:36

Beside duplicating the point layer or fetching the desired attributes in the Field Calculator, you can also achieve this goal using a "Virtual layer"

Using this approach, you can also compute your new field.

Go to the menu Layer > Add Layer > Add/Edit Virtual Layer... and enter the following query. Adjust the layer and field names.

Remember that virtual layers are computed in memory, so feel free to export it to another format if needed.

SELECT l.*, 
      pt1.label as label_origin, pt2.label as label_extreme, 
      pt1.label || ' To ' || pt2.label
FROM myLineLayer l
JOIN myPointLayer pt1
 ON l.origin_id = pt1.id
JOIN myPointLayer pt2
 ON l.extreme_id = pt2.id;

Thanks to you JGH, I learned this new trick ;) – Timeless. Jan 02 '23 at 13:37 — Timeless., Jan 02 '23 at 13:37

score 2 · Answer 3 · answered Jan 02 '23 at 15:33

Another solution is based on PyQGIS. It utilizes the QgsVectorLayerJoinInfo() class for joining purposes.

Proceed with Plugins > Python Console > Show Editor and paste the script below:

# imports
from qgis.core import QgsProject, QgsVectorLayerJoinInfo
original layer
layer = QgsProject.instance().mapLayersByName("union")[0]
field = 'id'
target layer to join
target_layer = QgsProject.instance().mapLayersByName("points")[0]
target_layer_id = target_layer.id()
params for the first join
target_field1 = 'from'
prefix1 = 'from_'
params for the second join
target_field2 = 'to'
prefix2 = 'to_'
First object to join
joinObject1 = QgsVectorLayerJoinInfo()
joinObject1.setJoinFieldName(field)
joinObject1.setTargetFieldName(target_field1)
joinObject1.setJoinLayerId(target_layer_id)
joinObject1.setJoinFieldNamesSubset(["date"])
joinObject1.setPrefix(prefix1)
joinObject1.setUsingMemoryCache(True)
joinObject1.setJoinLayer(target_layer)
layer.addJoin(joinObject1)
Second object to join
joinObject2 = QgsVectorLayerJoinInfo()
joinObject2.setJoinFieldName(field)
joinObject2.setTargetFieldName(target_field2)
joinObject2.setJoinLayerId(target_layer_id)
joinObject2.setJoinFieldNamesSubset(["date"])
joinObject2.setPrefix(prefix2)
joinObject2.setUsingMemoryCache(True)
joinObject2.setJoinLayer(target_layer)
layer.addJoin(joinObject2)