Ah, I had no idea you could bind python functions into sqlite!

I think the primary purpose of this issue has been served now - I'm going to close this and create a new issue for the only bit of this that hasn't been touched yet, which is (optionally) exposing units in the JSON API.

I built a prototype of the convert_units() custom SQL function as a plugin over in https://github.com/simonw/datasette/issues/14#issuecomment-381446511

I think I'm going to hold on to the custom sql function idea for the moment and implement it as an example plugin.

Presumably units only work for numeric fields? If that's the case then automatically processing them if the incoming query string argument has a unit suffix makes total sense to me.

Here's a pretty crazy idea: what if we exposed unit conversion to SQL as a custom SQLite function? That way it would be possible to optionally use units in actual custom SQL queries.

I'd have to think quite carefully about performance implications here - wouldn't want a poorly considered unit calculation over a 500,000 row table to lock up the server. But I think the 1s query time limit might still prevent that.

I'd like to figure out a sensible opt-in way to expose this in the JSON output as well. Maybe with a &_units=true parameter?

<s>From a machine-readable perspective I'm not sure why it would be useful to decorate the values with units</s>. Edit: Should have had some coffee first. It's clearly useful for stuff like map rendering!

I agree that the unit metadata should definitely be exposed in the JSON.

In #204 you said "I'd like to add support for using units when querying but this is PR is pretty usable as-is." - I'm fascinated to hear more about how this could work.

I'm thinking about a couple of approaches here. I think the simplest one is: if the column has a unit attached, optionally accept units in query fields:

```python column_units = ureg("Hz") # Create a unit object for the column's unit query_variable = ureg("4 GHz") # Supplied query variable

Now we can convert the query units into column units before querying

supplied_value.to(column_units).magnitude

4000000000.0

If the user doesn't supply units, pint just returns the plain

number and we can query as usual assuming it's the base unit

query_variable = ureg("50") query_variable

50

isinstance(query_variable, numbers.Number)

True ```

This also lets us do some nice unit conversion on querying:

```python column_units = ureg("m") query_variable = ureg("50 ft")

supplied_value.to(column_units)

<Quantity(15.239999999999998, 'meter')> ```

The alternative would be to provide a dropdown of units next to the query field (so a "Hz" field would give you "kHz", "MHz", "GHz"). Although this would be clearer to the user, it isn't so easy - we'd need to know more about the context of the field to give you sensible SI prefixes (I'm not so interested in nanoHertz, for example).

You also lose the bonus of being able to convert - although pint will happily show you all the compatible units, it again suffers from a lack of context:

```python ureg("m").compatible_units()

frozenset({<Unit('angstrom')>, <Unit('thou')>, <Unit('inch')>, <Unit('link')>, <Unit('foot')>, <Unit('survey_foot')>, <Unit('yard')>, <Unit('rod')>, <Unit('mile')>, <Unit('survey_mile')>, <Unit('league')>, <Unit('light_year')>}) ```

In #204 you said "I'd like to add support for using units when querying but this is PR is pretty usable as-is." - I'm fascinated to hear more about how this could work.

This is really cool - I'm very impressed by pint.

I'd like to figure out a sensible opt-in way to expose this in the JSON output as well. Maybe with a &_units=true parameter?

We should definitely expose the units section from the table metadata in the output of https://wtr-api.herokuapp.com/wtr-663ea99/license_frequency.json

Looks like pint is pretty good at this.

```python In [1]: import pint

In [2]: ureg = pint.UnitRegistry()

In [3]: q = 3e6 * ureg('Hz')

In [4]: '{:~P}'.format(q.to_compact()) Out[4]: '3.0 MHz'

In [5]: q = 0.3 * ureg('m')

In [5]: '{:~P}'.format(q.to_compact()) Out[5]: '300.0 mm'

In [6]: q = 5 * ureg('')

In [7]: '{:~P}'.format(q.to_compact()) Out[7]: '5' ```

This also feeds into the visualization features I want to add - we could use this kind of metadata to automatically apply meaningful labels to graphs.

I like this. I'd like to be able to attach a full description to a column as well. We could support these in metadata.json