mirror of
https://github.com/PaiGramTeam/sqlmodel.git
synced 2024-11-23 16:17:47 +00:00
309 lines
8.0 KiB
Markdown
309 lines
8.0 KiB
Markdown
# 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**.
|
|
|
|
<img class="shadow" alt="table with first 3 rows selected" src="/img/tutorial/offset-and-limit/limit.svg">
|
|
|
|
## 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:
|
|
|
|
```Python hl_lines="4-10"
|
|
# Code above omitted 👆
|
|
|
|
{!./docs_src/tutorial/offset_and_limit/tutorial001.py[ln:23-41]!}
|
|
|
|
# Code below omitted 👇
|
|
```
|
|
|
|
<details>
|
|
<summary>👀 Full file preview</summary>
|
|
|
|
```Python
|
|
{!./docs_src/tutorial/offset_and_limit/tutorial001.py!}
|
|
```
|
|
|
|
</details>
|
|
|
|
## Review Select All
|
|
|
|
This is the code we had to select all the heroes in the `select()` examples:
|
|
|
|
```Python hl_lines="3-8"
|
|
# Code above omitted 👆
|
|
|
|
{!./docs_src/tutorial/select/tutorial003.py[ln:36-41]!}
|
|
|
|
# Code below omitted 👇
|
|
```
|
|
|
|
<details>
|
|
<summary>👀 Full file preview</summary>
|
|
|
|
```Python
|
|
{!./docs_src/tutorial/select/tutorial003.py!}
|
|
```
|
|
|
|
</details>
|
|
|
|
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:
|
|
|
|
```Python hl_lines="5"
|
|
# Code above omitted 👆
|
|
|
|
{!./docs_src/tutorial/offset_and_limit/tutorial001.py[ln:44-49]!}
|
|
|
|
# Code below omitted 👇
|
|
```
|
|
|
|
<details>
|
|
<summary>👀 Full file preview</summary>
|
|
|
|
```Python
|
|
{!./docs_src/tutorial/offset_and_limit/tutorial001.py!}
|
|
```
|
|
|
|
</details>
|
|
|
|
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.
|
|
|
|
<img class="shadow" alt="table with first 3 rows selected" src="/img/tutorial/offset-and-limit/limit.svg">
|
|
|
|
## Run the Program on the Command Line
|
|
|
|
If we run it on the command line, it will output:
|
|
|
|
<div class="termy">
|
|
|
|
```console
|
|
$ 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')
|
|
]
|
|
```
|
|
|
|
</div>
|
|
|
|
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?
|
|
|
|
<img class="shadow" alt="table with next rows selected, from 4 to 6" src="/img/tutorial/offset-and-limit/limit2.svg">
|
|
|
|
We can use `.offset()`:
|
|
|
|
```Python hl_lines="5"
|
|
# Code above omitted 👆
|
|
|
|
{!./docs_src/tutorial/offset_and_limit/tutorial002.py[ln:44-49]!}
|
|
|
|
# Code below omitted 👇
|
|
```
|
|
|
|
<details>
|
|
<summary>👀 Full file preview</summary>
|
|
|
|
```Python
|
|
{!./docs_src/tutorial/offset_and_limit/tutorial002.py!}
|
|
```
|
|
|
|
</details>
|
|
|
|
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 Command Line
|
|
|
|
Now we can run the program on the command line, and it will output:
|
|
|
|
<div class="termy">
|
|
|
|
```console
|
|
$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')
|
|
]
|
|
```
|
|
|
|
</div>
|
|
|
|
## Select Next Batch
|
|
|
|
Then to get the next batch of 3 rows we would offset all the ones we already saw, the first 6:
|
|
|
|
```Python hl_lines="5"
|
|
# Code above omitted 👆
|
|
|
|
{!./docs_src/tutorial/offset_and_limit/tutorial003.py[ln:44-49]!}
|
|
|
|
# Code below omitted 👇
|
|
```
|
|
|
|
<details>
|
|
<summary>👀 Full file preview</summary>
|
|
|
|
```Python
|
|
{!./docs_src/tutorial/offset_and_limit/tutorial003.py!}
|
|
```
|
|
|
|
</details>
|
|
|
|
The database right now has **only 7 rows**, so this query can only get 1 row.
|
|
|
|
<img class="shadow" alt="table with the last row (7th) selected" src="/img/tutorial/offset-and-limit/limit3.svg">
|
|
|
|
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 have to find that many results.
|
|
|
|
## Run the Program with the Last Batch on the Command Line
|
|
|
|
And if we run it in the command line, it will output:
|
|
|
|
<div class="termy">
|
|
|
|
```console
|
|
$ 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')
|
|
]
|
|
```
|
|
|
|
</div>
|
|
|
|
## 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:
|
|
|
|
```SQL
|
|
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:
|
|
|
|
<img class="shadow" alt="DB Browser for SQLite showing the result of the SQL query" src="/img/tutorial/offset-and-limit/db-browser.png">
|
|
|
|
## 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:
|
|
|
|
```Python hl_lines="5"
|
|
# Code above omitted 👆
|
|
|
|
{!./docs_src/tutorial/offset_and_limit/tutorial004.py[ln:44-49]!}
|
|
|
|
# Code below omitted 👇
|
|
```
|
|
|
|
<details>
|
|
<summary>👀 Full file preview</summary>
|
|
|
|
```Python
|
|
{!./docs_src/tutorial/offset_and_limit/tutorial004.py!}
|
|
```
|
|
|
|
</details>
|
|
|
|
## Run the Program with Limit and Where on the Command 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:
|
|
|
|
<div class="termy">
|
|
|
|
```console
|
|
$ 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')
|
|
]
|
|
```
|
|
|
|
</div>
|
|
|
|
## 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()`.
|