Group the Joins and Relations sections in a dedicated chapter #8435

DelazJ · 2023-08-11T19:47:22Z

Supersedes #7563

The last section, about widgets and forms, is not in the PR. Not yet sure what existing sections to move there and how to structure it but the idea is to have most of the form widgets that handle relations and joins described also in this chapter.

Fixes #7558, fixes #6205, refs #7709

No plans to backport!

DelazJ · 2023-08-11T19:48:16Z

@ponceta ?

ponceta · 2023-08-14T08:45:26Z

I'm a great fan of this! 👍

@3nids do you think this is a good way to go?

I got quickly over the builds artifacts and it looks good.

https://github.com/qgis/QGIS-Documentation/actions/runs/5836463495

DelazJ · 2023-09-01T07:59:09Z

@3nids, may I? Thanks

3nids

good for me, some minor remarks

docs/user_manual/introduction/qgis_configuration.rst

docs/user_manual/working_with_vector/joins_relations.rst

3nids · 2023-09-01T12:43:16Z

docs/user_manual/working_with_vector/joins_relations.rst

+**Joins** allow you to associate features of the current layer
+to features from another loaded vector layer.
+Whether they are spatially enabled and their geometry type does not matter.
+The join is based on an attribute that is shared by the layers, in a one-to-one relationship.


it's not always one-to-one I believe. One-to-many is also possible.

Now that I read again, I think I wasn't clear enough. I was referring to joins in QGIS (rewording suggested above). Is that possible to have a one-to-many join in QGIS? Does QGIS not only perform a left join? I'd be curious for the how-to.

As I understand it, QGIS doesn't have a direct built-in one-to-many join operation for attribute tables. To achieve one-to-many relationship-like results, users need to use SQL queries or spatial databases. Therefore, there is no single workflow in QGIS for one-to-many joins. Consequently, I don't think one-to-many joins can be adequately described without external guidelines (such as PostgreSQL/PostGIS), and I'm not sure if that's something we want to include in QGIS documentation. @DelazJ, you may have more insight into the rules for external guidelines in QGIS docs.

Anyway, while I'm not an expert on spatial databases, I do know that Joins in QGIS allow relationships where features in one layer can be associated with features in another layer based on shared attributes. For instance, you could join a point layer representing cities to a table containing information about each city. Each city point can be associated with one record in the attribute table. Perhaps this is considered a one-to-many join, even if it's not a true one-to-many relationship in a relational database sense. In other words, is it possible that the terms 'one-to-one' and 'one-to-many' are misunderstood?

@selmaVH1 Thanks for the feedback and confirming that joins in layer properties only allow a feature to be associated to one other record.
Speaking of one-to-many relation, this is also possible in QGIS through the project relations but this is already addressed in the other part of the PR.

3nids · 2023-09-01T12:45:40Z

docs/user_manual/working_with_vector/joins_relations.rst

+QGIS currently has support for joining non-spatial table formats supported by GDAL
+(e.g., CSV, DBF and Excel), delimited text and the PostgreSQL providers.
+
+.. is the above still true? No more supported formats (oracle, mssql, ...)???


I would have thought that this is available for any data source.

3nids · 2023-09-01T12:47:33Z

docs/user_manual/working_with_vector/joins_relations.rst

+Setting relations between multiple layers
+=========================================
+
+Unlike joins that define a one-to-one link between features across two layers,


I don't think this is the difference (as stated above, joins are not strictly 1-to-1). To me the main difference is that relation offers a mean to navigate through the relations, in other words list and discover features which are associated to each other.

docs/user_manual/working_with_vector/joins_relations.rst

3nids · 2023-09-01T12:52:57Z

docs/user_manual/working_with_vector/joins_relations.rst

+
+  You should enable the transaction mode in :menuselection:`Project Properties --> Data Sources -->`
+  when working on such context.
+  QGIS should be able to add or update row(s) in all tables (airlines, airports and the pivot tables).


shall we suggest here to use UUIDs instead of increment?

Sorry but I have no idea what you mean, @3nids

@3nids I assume this refers to PostgreSQL, not QGIS?

docs/user_manual/working_with_vector/vector_properties.rst

DelazJ

Thanks @3nids for the feedback
I tried to reply and integrate your various comments (as suggestions for the moment instead of commits). I'm honestly quite unsure of the joins IN QGIS allowing one-to-many cardinality so if you can share some evidences, I will (be unhappy to have to) refactor the PR accordingly.

docs/user_manual/introduction/qgis_configuration.rst

DelazJ · 2023-09-04T08:46:20Z

docs/user_manual/working_with_vector/joins_relations.rst

+**Joins** allow you to associate features of the current layer
+to features from another loaded vector layer.
+Whether they are spatially enabled and their geometry type does not matter.
+The join is based on an attribute that is shared by the layers, in a one-to-one relationship.


Now that I read again, I think I wasn't clear enough. I was referring to joins in QGIS (rewording suggested above). Is that possible to have a one-to-many join in QGIS? Does QGIS not only perform a left join? I'd be curious for the how-to.

DelazJ · 2023-09-04T08:50:54Z

docs/user_manual/working_with_vector/joins_relations.rst

+
+  You should enable the transaction mode in :menuselection:`Project Properties --> Data Sources -->`
+  when working on such context.
+  QGIS should be able to add or update row(s) in all tables (airlines, airports and the pivot tables).


Sorry but I have no idea what you mean, @3nids

docs/user_manual/working_with_vector/vector_properties.rst

docs/user_manual/working_with_vector/joins_relations.rst

DelazJ · 2023-10-17T01:23:28Z

@selmaVH1 May I request your review here, particularly on unresolved points raised by @3nids? Thanks.

Co-authored-by: Denis Rouzaud <denis.rouzaud@gmail.com>

ponceta · 2023-12-22T10:28:28Z

Many many thanks to you! This is a great improvement for the understanding of relation management in QGIS! 👍

DelazJ added Project/Configuration project properties, global options and settings Properties labels Aug 11, 2023

DelazJ mentioned this pull request Aug 11, 2023

[WIP] Work with relations #7563

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3nids approved these changes Sep 1, 2023

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DelazJ commented Sep 14, 2023

View reviewed changes

ponceta and others added 8 commits October 23, 2023 15:30

Initial restructuring of working with relations

3929c2d

Use a legible and meaningful name for relation-related screenshots

eb0eb04

Move the Joins properties section to the new chapter

9f78a2d

Add some clarification to the join process

fee1eb8

Simplify relations sectioning and adjust description with dataset

17e77dc

Describe buttons of the relations tab

d1c053a

Rework and contextualize joins and relations in GIS

51ad93a

Apply suggestions from review

8fba947

Co-authored-by: Denis Rouzaud <denis.rouzaud@gmail.com>

DelazJ force-pushed the workingWithRelations branch from c2f5f73 to 8fba947 Compare October 23, 2023 13:36

DelazJ requested a review from selmaVH1 October 28, 2023 06:31

DelazJ added the backport release_3.34 On merge create a backported pull request to 3.34 label Dec 20, 2023

DelazJ merged commit f021603 into qgis:master Dec 22, 2023
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DelazJ deleted the workingWithRelations branch December 22, 2023 08:45

qgis-bot mentioned this pull request Dec 22, 2023

[Backport release_3.34] Group the Joins and Relations sections in a dedicated chapter #8724

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DelazJ mentioned this pull request Mar 1, 2024

Moving "Storing and fetching an external resource" section… #8911

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Group the Joins and Relations sections in a dedicated chapter #8435

Group the Joins and Relations sections in a dedicated chapter #8435

DelazJ commented Aug 11, 2023

DelazJ commented Aug 11, 2023

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DelazJ commented Oct 17, 2023

ponceta commented Dec 22, 2023

Group the Joins and Relations sections in a dedicated chapter #8435

Group the Joins and Relations sections in a dedicated chapter #8435

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DelazJ commented Aug 11, 2023

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DelazJ commented Oct 17, 2023

ponceta commented Dec 22, 2023