Django ORM if you already know SQL

If you are migrating to Django from another MVC framework, chances are you already know SQL.

In this post, I will be illustrating how to use Django ORM by drawing analogies to equivalent SQL statements. Connecting a new topic to your existing knowledge will help you learn to use the ORM faster.

Let us consider a simple base model for a person with attributes name, age, and gender.

Person ER Diagram

To implement the above entity, we would model it as a table in SQL.

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CREATE TABLE Person (
    id int,
    name varchar(50),
    age int NOT NULL,
    gender varchar(10),
);

The same table is modeled in Django as a class which inherits from the base Model class. The ORM creates the equivalent table under the hood.

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class Person(models.Model):
    name = models.CharField(max_length=50, blank=True)
    age = models.IntegerField()
    gender = models.CharField(max_length=10, blank=True)

The most used data types are:

SQL Django
INT IntegerField()
VARCHAR(n) CharField(max_length=n)
TEXT TextField()
FLOAT(n) FloatField()
DATE DateField()
TIME TimeField()
DATETIME DateTimeField()

The various queries we can use are:

SELECT Statement

Fetch all rows
SQL:

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SELECT *
FROM Person;

Django:

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persons = Person.objects.all()
for person in persons:
    print(person.name)
    print(person.gender)
    print(person.age)

Fetch specific columns
SQL:

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SELECT name, age
FROM Person;

Django:

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Person.objects.only('name', 'age')

Fetch distinct rows
SQL:

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SELECT DISTINCT name, age
FROM Person;

Django:

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Person.objects.values('name', 'age').distinct()

Fetch specific number of rows
SQL:

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SELECT *
FROM Person
LIMIT 10;

Django:

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Person.objects.all()[:10]

LIMIT AND OFFSET keywords
SQL:

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SELECT *
FROM Person
OFFSET 5
LIMIT 5;

Django:

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Person.objects.all()[5:10]

WHERE Clause

Filter by single column
SQL:

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SELECT *
FROM Person
WHERE id = 1;

Django:

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Person.objects.filter(id=1)

Filter by comparison operators
SQL:

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WHERE age > 18;
WHERE age >= 18;
WHERE age < 18;
WHERE age <= 18;
WHERE age != 18;

Django:

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Person.objects.filter(age__gt=18)
Person.objects.filter(age__gte=18)
Person.objects.filter(age__lt=18)
Person.objects.filter(age__lte=18)
Person.objects.exclude(age=18)

BETWEEN Clause
SQL:

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SELECT *
FROM Person 
WHERE age BETWEEN 10 AND 20;

Django:

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Person.objects.filter(age__range=(10, 20))

LIKE operator
SQL:

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WHERE name like '%A%';
WHERE name like binary '%A%';
WHERE name like 'A%';
WHERE name like binary 'A%';
WHERE name like '%A';
WHERE name like binary '%A';

Django:

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Person.objects.filter(name__icontains='A')
Person.objects.filter(name__contains='A')
Person.objects.filter(name__istartswith='A')
Person.objects.filter(name__startswith='A')
Person.objects.filter(name__iendswith='A')
Person.objects.filter(name__endswith='A')

IN operator
SQL:

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WHERE id in (1, 2);

Django:

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Person.objects.filter(id__in=[1, 2])

AND, OR and NOT Operators

SQL:

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WHERE gender='male' AND age > 25;

Django:

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Person.objects.filter(gender='male', age__gt=25)

SQL:

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WHERE gender='male' OR age > 25;

Django:

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from django.db.models import Q
Person.objects.filter(Q(gender='male') | Q(age__gt=25))

SQL:

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WHERE NOT gender='male';

Django:

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Person.objects.exclude(gender='male')

NULL Values

SQL:

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WHERE age is NULL;
WHERE age is NOT NULL;

Django:

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Person.objects.filter(age__isnull=True)
Person.objects.filter(age__isnull=False)

# Alternate approach
Person.objects.filter(age=None)
Person.objects.exclude(age=None)

ORDER BY Keyword

Ascending Order
SQL:

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SELECT *
FROM Person
order by age;

Django:

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Person.objects.order_by('age')

Descending Order
SQL:

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SELECT *
FROM Person
ORDER BY age DESC;

Django:

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Person.objects.order_by('-age')

INSERT INTO Statement

SQL:

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INSERT INTO Person
VALUES ('Jack', '23', 'male');

Django:

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Person.objects.create(name='jack', age=23, gender='male)

UPDATE Statement

Update single row
SQL:

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UPDATE Person
SET age = 20
WHERE id = 1;

Django:

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person = Person.objects.get(id=1)
person.age = 20
person.save()

Update multiple rows
SQL:

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UPDATE Person
SET age = age * 1.5;

Django:

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from django.db.models import F

Person.objects.update(age=F('age')*1.5)

DELETE Statement

Delete all rows
SQL:

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DELETE FROM Person;

Django:

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Person.objects.all().delete()

Delete specific rows
SQL:

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DELETE FROM Person
WHERE age < 10;

Django:

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Person.objects.filter(age__lt=10).delete()

Aggregation

MIN Function
SQL:

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SELECT MIN(age)
FROM Person;

Django:

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>>> from django.db.models import Min
>>> Person.objects.all().aggregate(Min('age'))
{'age__min': 0}

MAX Function
SQL:

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SELECT MAX(age)
FROM Person;

Django:

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>>> from django.db.models import Max
>>> Person.objects.all().aggregate(Max('age'))
{'age__max': 100}

AVG Function
SQL:

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SELECT AVG(age)
FROM Person;

Django:

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>>> from django.db.models import Avg
>>> Person.objects.all().aggregate(Avg('age'))
{'age__avg': 50}

SUM Function
SQL:

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SELECT SUM(age)
FROM Person;

Django:

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>>> from django.db.models import Sum
>>> Person.objects.all().aggregate(Sum('age'))
{'age__sum': 5050}

COUNT Function
SQL:

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SELECT COUNT(*)
FROM Person;

Django:

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Person.objects.count()

GROUP BY Statement

Count of Person by gender
SQL:

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SELECT gender, COUNT(*) as count
FROM Person
GROUP BY gender;

Django:

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Person.objects.values('gender').annotate(count=Count('gender'))

HAVING Clause

Count of Person by gender if number of person is greater than 1
SQL:

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SELECT gender, COUNT('gender') as count
FROM Person
GROUP BY gender
HAVING count > 1;

Django:

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Person.objects.annotate(count=Count('gender'))
.values('gender', 'count')
.filter(count__gt=1)

JOINS

Consider a foreign key relationship between books and publisher.

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class Publisher(models.Model):
    name = models.CharField(max_length=100)

class Book(models.Model):
    publisher = models.ForeignKey(Publisher, on_delete=models.CASCADE)

Fetch publisher name for a book
SQL:

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SELECT name
FROM Book
LEFT JOIN Publisher
ON Book.publisher_id = Publisher.id
WHERE Book.id=1;

Django:

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book = Book.objects.select_related('publisher').get(id=1)
book.publisher.name

Fetch books which have specific publisher
SQL:

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SELECT *
FROM Book
WHERE Book.publisher_id = 1;

Django:

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publisher = Publisher.objects.prefetch_related('book_set').get(id=1)
books = publisher.book_set.all()

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