How to Group Data Using Django's Querysets?

Grouping Data in Django Queries

In Django, fetching data from the database often involves using querysets. These querysets provide various methods for filtering, ordering, and manipulating data. One common operation is grouping data by a specific field, similar to the SQL GROUP BY clause.

Aggregation for Grouping

To group data in Django, one can utilize the aggregation features of the ORM. For instance, consider the following query that retrieves all members:

Members.objects.all()

This query returns a list of tuples, each tuple representing a member's details:

[('Eric', 'Salesman', 'X-Shop'),
 ('Freddie', 'Manager', 'X2-Shop'),
 ('Teddy', 'Salesman', 'X2-Shop'),
 ('Sean', 'Manager', 'X2-Shop')]

To group these results by the designation field, one can use the values() and annotate() methods:

from django.db.models import Count

result = (
    Members.objects
    .values('designation')
    .annotate(dcount=Count('designation'))
    .order_by()
)

This query generates a SQL statement similar to:

SELECT designation, COUNT(designation) AS dcount
FROM members
GROUP BY designation

The result is a list of dictionaries, each representing a designation and the count of members for that designation:

[{'designation': 'Salesman', 'dcount': 2}, {'designation': 'Manager', 'dcount': 2}]

To include multiple fields in the results, simply add them as arguments to the values() method, as shown here:

.values('designation', 'first_name', 'last_name')

References

For more information on aggregation and grouping in Django, refer to the following resources:

• Django documentation: [Values](https://docs.djangoproject.com/en/stable/ref/models/querysets/#values), [Annotate](https://docs.djangoproject.com/en/stable/ref/models/querysets/#annotate), [Count](https://docs.djangoproject.com/en/stable/ref/models/querysets/#django.db.models.Count)
• Django documentation: [Aggregation](https://docs.djangoproject.com/en/stable/topics/db/aggregation/), especially the section on [Interaction with Default Ordering or order_by()](https://docs.djangoproject.com/en/stable/topics/db/aggregation/#interaction-with-default-ordering-or-orderby)

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