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The group_by parameter aggregates entities into groups and counts how many are in each group. 
# Count works by type
https://api.openalex.org/works?group_by=type
Response: 
{
  "meta": {
    "count": 246136992,
    "groups_count": 15
  },
  "group_by": [
    {
      "key": "article",
      "key_display_name": "article",
      "count": 202814957
    },
    {
      "key": "book-chapter",
      "key_display_name": "book-chapter",
      "count": 21250659
    },
    {
      "key": "dissertation",
      "key_display_name": "dissertation",
      "count": 6055973
    }
  ]
}

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Group properties

Each group object contains:
PropertyDescription
keyThe OpenAlex ID or raw value for this group
key_display_nameThe display name or raw value for this group
countNumber of entities in the group

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key vs key_display_name

When grouping by an OpenAlex entity, key is the ID and key_display_name is the display name: 
# Group works by institution
https://api.openalex.org/works?group_by=authorships.institutions.id
Returns groups like: 
{
  "key": "https://openalex.org/I136199984",
  "key_display_name": "Harvard University",
  "count": 1234567
}
For non-entity values (like level), both key and key_display_name are the raw value.

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Including unknowns

The “unknown” group (entities without a value) is hidden by default. Add :include_unknown to include it: 
https://api.openalex.org/works?group_by=authorships.countries

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How “unknown” is keyed

The bucket’s key depends on the field type:
Field typeExampleUnknown key
String / IDprimary_location.license, type"unknown"
Numeric (range)apc_paid.value, cited_by_count-111 (sentinel)
Booleanis_oareturned as the false bucket
Numeric fields use the sentinel -111 because the aggregation bucket can’t mix a string key like "unknown" into a numeric series. Treat any -111 (or -111.0) bucket key as “unknown.”

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What counts as “unknown”

A work is counted in the unknown bucket when the underlying field is missing or empty. For fields nested inside an array — for example authorships.institutions.type — the work is also counted as unknown when the array is present but empty (e.g. an authorship with no institutions). If you reproduce group_by counts in code by iterating the JSON, treat empty lists the same way to match the API’s totals.

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Combining with filters

Group by and filter work together: 
# Count work types for 2023 publications
https://api.openalex.org/works?filter=publication_year:2023&group_by=type

# Count open access status for a specific author
https://api.openalex.org/works?filter=author.id:A5023888391&group_by=open_access.is_oa

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Paging

A grouped response returns at most 200 groups per page. To page through more, use cursor paging: start with cursor=* and follow meta.next_cursor until it’s null, exactly as you would for a list of results. 
# First page of groups
https://api.openalex.org/works?group_by=authorships.author.id&per_page=200&cursor=*

# Next page: use the next_cursor value from the previous response
https://api.openalex.org/works?group_by=authorships.author.id&per_page=200&cursor=<next_cursor>
Basic page-based paging (page=2, …) is not supported with group_by — use cursor paging. When paging through groups, results are sorted by key (not by count).

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Meta properties

PropertyDescription
meta.countTotal entities matching the filter
meta.groups_countNumber of groups in current page (or null if none)
Due to technical limitations, groups_count only reports groups in the current page, not the total number of groups.