Our WIP script: \a[wikipedia/import-categories.sh].

Related:
* https://opendata.stackexchange.com/questions/1533/download-wikipedia-articles-from-a-specific-category
* https://webapps.stackexchange.com/questions/16359/is-there-a-way-to-download-a-list-of-all-wikipedia-categories/172480\#172480
* https://stackoverflow.com/questions/40119322/how-to-download-all-pages-inside-a-category-in-wikipedia
* category tree on Stack Overflow
  * https://stackoverflow.com/questions/17432254/wikipedia-category-hierarchy-from-dumps/77313490\#77313490 Canon but no good answers.
  * https://stackoverflow.com/questions/12227134/how-to-fetch-category-tree-of-wiki
  * https://stackoverflow.com/questions/21782410/finding-subcategories-of-a-wikipedia-category-using-category-and-categorylinks-t[]. Actually explains it: https://stackoverflow.com/questions/21782410/finding-subcategories-of-a-wikipedia-category-using-category-and-categorylinks-t/21798259\#21798259
  * https://stackoverflow.com/questions/27279649/how-to-build-wikipedia-category-hierarchy
* https://mdkzaman.com/knowledge-graph-from-wikipedia-category-hierarchy/

Consider:
* https://en.wikipedia.org/wiki/Category:Computer_storage_devices
* https://en.wikipedia.org/wiki/Category:Computer_data_storage
* https://en.wikipedia.org/wiki/Computer_storage_devices which redirects to: https://en.wikipedia.org/wiki/Computer_data_storage

Jewish_physicists

Let's observe them in <MySQL>:
``
mysql enwiki -e "select page_id, page_namespace, page_title, page_is_redirect from page where page_namespace in (0, 14) and page_title in ('Computer_storage_devices', 'Computer_data_storage')"
``
outputs:
``
+----------+----------------+--------------------------+------------------+
| page_id  | page_namespace | page_title               | page_is_redirect |
+----------+----------------+--------------------------+------------------+
|     5300 |              0 | Computer_data_storage    |                0 |
| 42371130 |              0 | Computer_storage_devices |                1 |
|   711721 |             14 | Computer_data_storage    |                0 |
|   895945 |             14 | Computer_storage_devices |                0 |
+----------+----------------+--------------------------+------------------+
``

``
mysql enwiki -e "select cl_from, cl_to from categorylinks where cl_from in (5300, 711721, 895945, 42371130)"
``
gives:
``
+----------+-----------------------------------------------------------------------+
| cl_from  | cl_to                                                                 |
+----------+-----------------------------------------------------------------------+
|     5300 | All_articles_containing_potentially_dated_statements                  |
|     5300 | Articles_containing_potentially_dated_statements_from_2009            |
|     5300 | Articles_containing_potentially_dated_statements_from_2011            |
|     5300 | Articles_with_GND_identifiers                                         |
|     5300 | Articles_with_NKC_identifiers                                         |
|     5300 | Articles_with_short_description                                       |
|     5300 | Computer_architecture                                                 |
|     5300 | Computer_data_storage                                                 |
|     5300 | Short_description_matches_Wikidata                                    |
|     5300 | Use_dmy_dates_from_June_2020                                          |
|     5300 | Wikipedia_articles_incorporating_text_from_the_Federal_Standard_1037C |
|   711721 | Computer_architecture                                                 |
|   711721 | Computer_data                                                         |
|   711721 | Computer_hardware_by_type                                             |
|   711721 | Data_storage                                                          |
|   895945 | Computer_data_storage                                                 |
|   895945 | Computer_peripherals                                                  |
|   895945 | Recording_devices                                                     |
| 42371130 | Redirects_from_alternative_names                                      |
+----------+-----------------------------------------------------------------------+
``

So we see that `cl_from` encodes the parent categories:
* parent categories of categories:
  * https://en.wikipedia.org/wiki/Category:Computer_data_storage[], which has ID `711721`, has parent categories: "Computer hardware by type", "Computer data", "Data storage", "Computer architecture". This matches exactly on the database. These are all encoded on the source code of the page:
    ``
    {{DEFAULTSORT:Storage}}
    [[Category:Computer hardware by type]]
    [[Category:Computer data|Storage]]
    [[Category:Data storage|Computer]]
    [[Category:Computer architecture]]
    ``
  * https://en.wikipedia.org/wiki/Category:Computer_storage_devices[] has parent categories: "Computer data storage", "Recording devices", "Computer peripherals". This matches exactly on the database.
* parent categories of pages:
  * https://en.wikipedia.org/wiki/Computer_storage_devices whish is a redirect gets the magic category "Redirects_from_alternative_names", a humongous placeholder with many thousands of pages: https://en.wikipedia.org/wiki/Category:Redirects_from_alternative_names
  * https://en.wikipedia.org/wiki/Computer_data_storage shows only two categories onthe web UI: "Computer data storage" and "Computer architecture". Both of these are present on the database and at the end of the source code:
    ``
    {{DEFAULTSORT:Computer Data Storage}}
    [[Category:Computer data storage| ]]
    [[Category:Computer architecture]]
    ``
    The others appear to be more magic. Two of them we can guess from the templates:
    ``
    {{short description|Storage of digital data readable by computers}}
    {{Use dmy dates|date=June 2020}}
    ``
    are likely `Use_dmy_dates_from_June_2020` and `Articles_with_short_description` but the rest is more magic and not necessarily present in-source.

So to find all articls and categories under a given category title, say https://en.wikipedia.org/wiki/Category:Mathematics we can run:
``
mariadb enwiki -e "select cl_from, cl_to, page_namespace, page_title from categorylinks inner join page on page_namespace in (0, 14) and cl_from = page_id and cl_to = 'Mathematics'"
``


Discussion on "Download all Wikipedia categories" by Ciro Santilli 32