Compare with Wikibase tabular data model (#1059)

Co-authored-by: Peter Desmet <peter.desmet.work@gmail.com>

content/docs/guides/mediawiki-tabular-data.md

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+---
+title: Comparison with MediaWiki Tabular Data
+sidebar:
+  order: 3
+---
+
+<table>
+  <tr>
+    <th>Authors</th>
+    <td>Jakob Voß</td>
+  </tr>
+</table>
+
+[MediaWiki](https://www.mediawiki.org/) is the software used to run Wikipedia and related projects of the Wikimedia Foundation, including the media file repository [Wikimedia Commons](https://commons.wikimedia.org/). Commons hosts mostly images but also some records with tabular data. The [MediaWiki Tabular Data Model](https://www.mediawiki.org/wiki/Help:Tabular_data) was inspired by Data Package version 1 but it slightly differs from current Data Package specification, as described below.
+
+## Property Comparison
+
+A [MediaWiki tabular data page](https://www.mediawiki.org/wiki/Help:Tabular_data) describes and contains an individual table of data similar to a [Data Resource](/standard/data-resource/) with inline tabular data. Both are serialized as JSON objects, but the former comes as a page with unique name in a MediaWiki instance (such as Wikimedia Commons).
+
+### Top-level Properties
+
+MediaWiki Tabular Data has three required and two optional top-level properties. Most of these properties map to corresponding properties of a Data Resource:
+
+| MediaWiki Tabular Data                                                                                                                     | Data Package Table Schema                                                             |
+| ------------------------------------------------------------------------------------------------------------------------------------------ | ------------------------------------------------------------------------------------- |
+| - (implied by page name)                                                                                                                   | [name](/standard/data-resource/#name) (required) is a string                          |
+| [description](https://www.mediawiki.org/wiki/Help:Tabular_data#Top-level_fields) (optional) is a localized string                          | [description](/standard/data-resource/#description) (optional) is a CommonMark string |
+| [data](https://www.mediawiki.org/wiki/Help:Tabular_data#Top-level_fields) (required)                                                       | [data](/standard/data-resource/#name) (optional)                                      |
+| [license](https://www.mediawiki.org/wiki/Help:Tabular_data#Top-level_fields) (required) is the string `CC0-1.0` or another known identifier | [licenses](/standard/data-resource/#licenses) (optional) is an array                  |
+| [schema](https://www.mediawiki.org/wiki/Help:Tabular_data#Top-level_fields) (required) as [described below](#schema-properties)            | [schema](/standard/data-resource/#schema) (optional) can have multiple forms          |
+| [sources](https://www.mediawiki.org/wiki/Help:Tabular_data#Top-level_fields) (optional) is a string with Wiki markup                       | [sources](/standard/data-resource/#sources) (optional) is an array of objects       |
+
+The differences are:
+
+- property `name` does not exist but can be implied from page name
+- property `description` and `sources` have another format
+- property `data` is always an array of arrays and [data types](#data-types) of individual values can differ
+- property `schema` is required but it differs in definion of [schema properties](#schema-properties)
+- there is no property `licenses` but `license` fixed to plain string value `CC0-1.0` (other license indicators may be possible)
+
+### Data Types
+
+Tabular Data supports four data types that overlap with [Table Schema data types](/standard/table-schema/#field-types):
+
+- `number` subset of Table Schema [number](/standard/table-schema/#number) (no `NaN`, `INF`, or `-INF`)
+- `boolean` same as Table Schema [boolean](/standard/table-schema/#boolean)
+- `string` subset of Table Schema [string](/standard/table-schema/#string) (limited to 400 characters at most and must not include `\n` or `\t`)
+- `localized ` refers to an object that maps language codes to strings with same limitations as `string` type.
+  This type is not supported in Table Schema.
+
+Individual values in a MediaWiki Tabular Data table can always be `null`, while in Table Schema you need to explicitly list values that should be considered missing in [schema.missingValues](/standard/table-schema/#missingValues).
+
+### Schema Properties
+
+The `schema` property of MediaWiki tabular contains an object with property `fields` just like [Table Schema](/standard/table-schema/) but no other properties are allowed. Elements of this array are like Table Schema [field descriptors](/standard/table-schema/#field) limited to three properties and different value spaces:
+
+| MediaWiki Tabular Data                                                                                                                   | Data Package Table Schema                                                                |
+| ---------------------------------------------------------------------------------------------------------------------------------------- | ---------------------------------------------------------------------------------------- |
+| [name](https://www.mediawiki.org/wiki/Help:Tabular_data#Top-level_fields) (required) must be a string matching `^[a-zA-Z_][a-zA-Z_0-9]*` | [name](/standard/table-schema/#name) (required) can be any string                        |
+| [type](https://www.mediawiki.org/wiki/Help:Tabular_data#Top-level_fields) (required) is one of the [Data Types above](#data-types)       | [type](/standard/table-schema/#type) (optional) with [different data types](#data-types) |
+| [title](https://www.mediawiki.org/wiki/Help:Tabular_data#Top-level_fields) (optional) is a localized string                              | [title](/standard/table-schema/#title) (optional) is a plain string                      |