Django ORM if you already know SQL

Published October 31, 2018 in django

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.

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.

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:

SELECT *
FROM Person;

Django:

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

Fetch specific columns
SQL:

SELECT name, age
FROM Person;

Django:

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

Fetch distinct rows
SQL:

SELECT DISTINCT name, age
FROM Person;

Django:

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

Fetch specific number of rows
SQL:

SELECT *
FROM Person
LIMIT 10;

Django:

Person.objects.all()[:10]

LIMIT AND OFFSET keywords
SQL:

SELECT *
FROM Person
OFFSET 5
LIMIT 5;

Django:

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

WHERE Clause

Filter by single column
SQL:

SELECT *
FROM Person
WHERE id = 1;

Django:

Person.objects.filter(id=1)

Filter by comparison operators
SQL:

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

Django:

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:

SELECT *
FROM Person 
WHERE age BETWEEN 10 AND 20;

Django:

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

LIKE operator
SQL:

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:

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:

WHERE id in (1, 2);

Django:

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

AND, OR and NOT Operators

SQL:

WHERE gender='male' AND age > 25;

Django:

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

SQL:

WHERE gender='male' OR age > 25;

Django:

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

SQL:

WHERE NOT gender='male';

Django:

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

NULL Values

SQL:

WHERE age is NULL;
WHERE age is NOT NULL;

Django:

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:

SELECT *
FROM Person
order by age;

Django:

Person.objects.order_by('age')

Descending Order
SQL:

SELECT *
FROM Person
ORDER BY age DESC;

Django:

Person.objects.order_by('-age')

INSERT INTO Statement

SQL:

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

Django:

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

UPDATE Statement

Update single row
SQL:

UPDATE Person
SET age = 20
WHERE id = 1;

Django:

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

Update multiple rows
SQL:

UPDATE Person
SET age = age * 1.5;

Django:

from django.db.models import F

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

DELETE Statement

Delete all rows
SQL:

DELETE FROM Person;

Django:

Person.objects.all().delete()

Delete specific rows
SQL:

DELETE FROM Person
WHERE age < 10;

Django:

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

Aggregation

MIN Function
SQL:

SELECT MIN(age)
FROM Person;

Django:

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

MAX Function
SQL:

SELECT MAX(age)
FROM Person;

Django:

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

AVG Function
SQL:

SELECT AVG(age)
FROM Person;

Django:

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

SUM Function
SQL:

SELECT SUM(age)
FROM Person;

Django:

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

COUNT Function
SQL:

SELECT COUNT(*)
FROM Person;

Django:

Person.objects.count()

GROUP BY Statement

Count of Person by gender
SQL:

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

Django:

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

HAVING Clause

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

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

Django:

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

JOINS

Consider a foreign key relationship between books and publisher.

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:

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

Django:

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

Fetch books which have specific publisher
SQL:

SELECT *
FROM Book
WHERE Book.publisher_id = 1;

Django:

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

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