8.0 KiB
Read a Range of Data - LIMIT and OFFSET
Now you know how to get a single row with .one(), .first(), and session.get().
And you also know how to get multiple rows while filtering them using .where().
Now let's see how to get only a range of results.
Create Data
We will continue with the same code as before, but we'll modify it a little the select_heroes() function to simplify the example and focus on what we want to achieve here.
Again, we will create several heroes to have some data to select from:
# Code above omitted 👆
{!./docs_src/tutorial/offset_and_limit/tutorial001.py[ln:23-41]!}
# Code below omitted 👇
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{!./docs_src/tutorial/offset_and_limit/tutorial001.py!}
Review Select All
This is the code we had to select all the heroes in the select() examples:
# Code above omitted 👆
{!./docs_src/tutorial/select/tutorial003.py[ln:36-41]!}
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{!./docs_src/tutorial/select/tutorial003.py!}
But this would get us all the heroes at the same time, in a database that could have thousands, that could be problematic.
Select with Limit
We currently have 7 heroes in the database. But we could as well have thousands, so let's limit the results to get only the first 3:
# Code above omitted 👆
{!./docs_src/tutorial/offset_and_limit/tutorial001.py[ln:44-49]!}
# Code below omitted 👇
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{!./docs_src/tutorial/offset_and_limit/tutorial001.py!}
The special select object we get from select() also has a method .limit() that we can use to limit the results to a certain number.
In this case, instead of getting all the 7 rows, we are limiting them to only get the first 3.
Run the Program on the Comamnd Line
If we run it on the command line, it will output:
$ python app.py
// Previous output omitted 🙈
// Select with LIMIT
INFO Engine SELECT hero.id, hero.name, hero.secret_name, hero.age
FROM hero
LIMIT ? OFFSET ?
INFO Engine [no key 0.00014s] (3, 0)
// Print the heroes received, only 3
[
Hero(age=None, secret_name='Dive Wilson', id=1, name='Deadpond'),
Hero(age=None, secret_name='Pedro Parqueador', id=2, name='Spider-Boy'),
Hero(age=48, secret_name='Tommy Sharp', id=3, name='Rusty-Man')
]
Great! We got only 3 heroes as we wanted.
!!! tip We will check out that SQL code more in a bit.
Select with Offset and Limit
Now we can limit the results to get only the first 3.
But imagine we are in a user interface showing the results in batches of 3 heroes at a time.
!!! tip This is commonly called "pagination". Because the user interface would normally show a "page" of a predefined number of heroes at a time.
And then you can interact with the user interface to get the next page, and so on.
How do we get the next 3?
We can use .offset():
# Code above omitted 👆
{!./docs_src/tutorial/offset_and_limit/tutorial002.py[ln:44-49]!}
# Code below omitted 👇
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{!./docs_src/tutorial/offset_and_limit/tutorial002.py!}
The way this works is that the special select object we get from select() has methods like .where(), .offset() and .limit().
Each of those methods applies the change in the internal special select statement object, and also return the same object, this way, we can continue using more methods on it, like in the example above that we use both .offset() and .limit().
Offset means "skip this many rows", and as we want to skip the ones we already saw, the first three, we use .offset(3).
Run the Program with Offset on the Comamnd Line
Now we can run the program on the command line, and it will output:
$python app.py
// Previous output omitted 🙈
// Select with LIMIT and OFFSET
INFO Engine SELECT hero.id, hero.name, hero.secret_name, hero.age
FROM hero
LIMIT ? OFFSET ?
INFO Engine [no key 0.00020s] (3, 3)
// Print the 3 heroes received, the second batch
[
Hero(age=32, secret_name='Natalia Roman-on', id=4, name='Tarantula'),
Hero(age=35, secret_name='Trevor Challa', id=5, name='Black Lion'),
Hero(age=36, secret_name='Steve Weird', id=6, name='Dr. Weird')
]
Select Next Batch
Then to get the next batch of 3 rows we would offset all the ones we already saw, the first 6:
# Code above omitted 👆
{!./docs_src/tutorial/offset_and_limit/tutorial003.py[ln:44-49]!}
# Code below omitted 👇
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{!./docs_src/tutorial/offset_and_limit/tutorial003.py!}
The database right now has only 7 rows, so this query can only get 1 row.
But don't worry, the database won't throw an error trying to get 3 rows when there's only one (as would happen with a Python list).
The database knows that we want to limit the number of results, but it doesn't necessarily has to find those many results.
Run the Program with the Last Batch on the Comamnd Line
And if we run it in the command line, it will output:
$ python app.py
// Previous output omitted 🙈
// Select last batch with LIMIT and OFFSET
INFO Engine SELECT hero.id, hero.name, hero.secret_name, hero.age
FROM hero
LIMIT ? OFFSET ?
INFO Engine [no key 0.00038s] (3, 6)
// Print last batch of heroes, only one
[
Hero(age=93, secret_name='Esteban Rogelios', id=7, name='Captain North America')
]
SQL with LIMIT and OFFSET
You probably noticed the new SQL keywords LIMIT and OFFSET.
You can use them in SQL, at the end of the other parts:
SELECT id, name, secret_name, age
FROM hero
LIMIT 3 OFFSET 6
If you try that in DB Browser for SQLite, you will get the same result:
Combine Limit and Offset with Where
Of course, you can also combine .limit() and .offset() with .where() and other methods you will learn about later:
# Code above omitted 👆
{!./docs_src/tutorial/offset_and_limit/tutorial004.py[ln:44-49]!}
# Code below omitted 👇
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{!./docs_src/tutorial/offset_and_limit/tutorial004.py!}
Run the Program with Limit and Where on the Comamnd Line
If we run it on the command line, it will find all the heroes in the database with an age above 32. That would normally be 4 heroes.
But we are limiting the results to only get the first 3:
$ python app.py
// Previous output omitted 🙈
// Select with WHERE and LIMIT
INFO Engine SELECT hero.id, hero.name, hero.secret_name, hero.age
FROM hero
WHERE hero.age > ?
LIMIT ? OFFSET ?
INFO Engine [no key 0.00022s] (32, 3, 0)
// Print the heroes received, only 3
[
Hero(age=35, secret_name='Trevor Challa', id=5, name='Black Lion'),
Hero(age=36, secret_name='Steve Weird', id=6, name='Dr. Weird'),
Hero(age=48, secret_name='Tommy Sharp', id=3, name='Rusty-Man')
]
Recap
Independently of how you filter the data with .where() or other methods, you can limit the query to get at maximum some number of results with .limit().
And the same way, you can skip the first results with .offset().