Download all Wikipedia categories

Our WIP script: wikipedia/import-categories.sh.

Consider:

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:
- 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]]
- 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:
- 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: en.wikipedia.org/wiki/Category:Redirects_from_alternative_names
- 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 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'"

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