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.
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()
Connect
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Top comments (3)
Thank you! That’s a really useful article.
Good article and it makes sense to me now that i know SQL again :)
Really good