A QuerySet in Django is a collection of SQL queries that Django’s Object-Relational Mapping (ORM) layer uses to interact with your database, abstracting the process of complex database queries. The Django web framework is designed around this concept, making it easy to translate complex relations and queries into Pythonic language, and vice versa.
QuerySets are fundamental in Django, they allow you to read, filter, limit, and manipulate the data stored in your database. You usually obtain a QuerySet by using your models‘ Manager. Each Django model has at least one Manager, and every time you’re calling ‘objects’, you’re actually referring to the default manager of a Django model which returns a new QuerySet object.
A very common use of QuerySets is for filtering data. For example, if you had a Django model named ‘Blog’, which could serve to represent blog posts, you could retrieve all the posts with ‘published’ status like this:
```
Blog.objects.filter(status=‘published’)
```
In this example, `Blog.objects` is referring to the default manager of the Blog model, and `.filter(status=‘published’)` is our query.
Retrieving specific objects is also highly efficient with QuerySets. Suppose you’d like to retrieve a blog post with an id of 5. You can do so as follows:
```
Blog.objects.get(id=5)
```
QuerySets are lazy, meaning they only fetch data from the database when needed. This is advantageous as it saves processing power and database resources.
Queries related to many-to-one relationships and many-to-many relationships can also be performed using Django QuerySets. Chains of queryset are also allowed because each one generates a new QuerySet that can be stored separately.
Additionally, QuerySets also support complex querying, ordering, slicing and aggregation. Django accomplishes this by translating Python code into SQL code, abstracting a lot of the database complexities away from you, the developer. Furthermore, Django keeps a history of SQL commands generated by previous QuerySets so that it can optimize and combine certain queries to minimize the database resources used.
Information used for the creation of this answer was retrieved from the official Django documentation (https://docs.djangoproject.com/en/3.2/topics/db/queries/) and Full Stack Python’s guide on Django ORM (https://www.fullstackpython.com/django-orm.html).
