# datasette-ersatz-table-valued-functions
[](https://pypi.org/project/datasette-ersatz-table-valued-functions/)
[](https://github.com/cldellow/datasette-ersatz-table-valued-functions/releases)
[](https://github.com/cldellow/datasette-ersatz-table-valued-functions/actions?query=workflow%3ATest)
[](https://github.com/cldellow/datasette-ersatz-table-valued-functions/blob/main/LICENSE)
**ersatz** *(adj.)*: made or used as a substitute, typically an inferior one, for something else.
Enable a limited form of table-valued functions.
See also [ersatz-table-valued-functions](https://github.com/cldellow/ersatz-table-valued-functions/).
## Installation
Install this plugin in the same environment as Datasette.
datasette install datasette-ersatz-table-valued-functions
## Usage
Write a plugin that registers a table-valued function in the `startup` hook:
```python
from datasette import hookimpl
from datasette_ersatz_table_valued_functions import create_table_function
def tbl_squares(n):
return [(i, i*i) for i in range(n)]
@hookimpl
def startup():
create_table_function('tbl_squares', 1, tbl_squares, ['root', 'square'])
```
You can then query this in Datasette:
```sql
SELECT root FROM tbl_squares(10) WHERE square % 2 = 0 AND square < 50
```
will emit 0, 2, 4, 6.
The parameters to the function can come from a subselect, which could target
any table and be arbitrarily complex:
```sql
SELECT root FROM tbl_squares((SELECT 10))
```
You can do whatever with the output of this function -- join it, aggregate it, etc.
What you can't do is use a join as the source of _input_ to the function:
```sql
WITH xs AS (SELECT 10 AS x) SELECT root FROM tbl_squares(x), xs
```
Queries that aren't supported are passed as-is to SQLite, which will itself
then reject them since no such table function is registered.
## Development
To set up this plugin locally, first checkout the code. Then create a new virtual environment:
cd datasette-ersatz-table-valued-functions
python3 -m venv venv
source venv/bin/activate
Now install the dependencies and test dependencies:
pip install -e '.[test]'
To run the tests:
pytest
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"description": "# datasette-ersatz-table-valued-functions\n\n[](https://pypi.org/project/datasette-ersatz-table-valued-functions/)\n[](https://github.com/cldellow/datasette-ersatz-table-valued-functions/releases)\n[](https://github.com/cldellow/datasette-ersatz-table-valued-functions/actions?query=workflow%3ATest)\n[](https://github.com/cldellow/datasette-ersatz-table-valued-functions/blob/main/LICENSE)\n\n**ersatz** *(adj.)*: made or used as a substitute, typically an inferior one, for something else.\n\nEnable a limited form of table-valued functions.\n\nSee also [ersatz-table-valued-functions](https://github.com/cldellow/ersatz-table-valued-functions/).\n\n## Installation\n\nInstall this plugin in the same environment as Datasette.\n\n datasette install datasette-ersatz-table-valued-functions\n\n## Usage\n\nWrite a plugin that registers a table-valued function in the `startup` hook:\n\n```python\nfrom datasette import hookimpl\nfrom datasette_ersatz_table_valued_functions import create_table_function\n\ndef tbl_squares(n):\n return [(i, i*i) for i in range(n)]\n\n@hookimpl\ndef startup():\n create_table_function('tbl_squares', 1, tbl_squares, ['root', 'square'])\n```\n\nYou can then query this in Datasette:\n\n```sql\nSELECT root FROM tbl_squares(10) WHERE square % 2 = 0 AND square < 50\n```\n\nwill emit 0, 2, 4, 6.\n\nThe parameters to the function can come from a subselect, which could target\nany table and be arbitrarily complex:\n\n```sql\nSELECT root FROM tbl_squares((SELECT 10))\n```\n\nYou can do whatever with the output of this function -- join it, aggregate it, etc.\n\nWhat you can't do is use a join as the source of _input_ to the function:\n\n```sql\nWITH xs AS (SELECT 10 AS x) SELECT root FROM tbl_squares(x), xs\n```\n\nQueries that aren't supported are passed as-is to SQLite, which will itself\nthen reject them since no such table function is registered.\n\n## Development\n\nTo set up this plugin locally, first checkout the code. Then create a new virtual environment:\n\n cd datasette-ersatz-table-valued-functions\n python3 -m venv venv\n source venv/bin/activate\n\nNow install the dependencies and test dependencies:\n\n pip install -e '.[test]'\n\nTo run the tests:\n\n pytest\n",
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