How to Select Rows with Duplicate Field Values in Django Using ORM and SQL?

Selecting Rows with Duplicate Field Values in Django

Consider a Django model with a non-unique name field that allows duplicate values. To select all rows with at least one duplicate value of the name field, one can explore both ORM and SQL approaches.

Django ORM Approach

Utilizing Django's aggregation capabilities, one can achieve this task with the following ORM expression:

from django.db.models import Count
Literal.objects.values('name')
               .annotate(Count('id')) 
               .order_by()
               .filter(id__count__gt=1)

This expression groups the model instances by name, counts the number of occurrences of each name, and filters the results to include only names with duplicate instances.

Enhanced Django ORM Approach

While the previous ORM approach provides the necessary functionality, it returns only the name and count information. To obtain the actual records, one can construct a regular QuerySet based on the duplicate names found:

dupes = Literal.objects.values('name')
                       .annotate(Count('id'))
                       .order_by()
                       .filter(id__count__gt=1)
Literal.objects.filter(name__in=[item['name'] for item in dupes])

SQL Approach

For those preferring a pure SQL solution, the following query would yield the desired results:

select * from literal where name IN (
    select name from literal group by name having count((name)) > 1
);

This query utilizes a subquery to find the duplicate names and then selects all rows from the main table where the name matches one of the duplicates.

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