Inline filters #82
Description
Something that's come up a couple of times (discussion of tertiary operator in https://github.com/max-sixty/prql/issues/11, Malloy impl in https://github.com/max-sixty/prql/issues/71) is the concept of inline filters:
Taken from Malloy's "filtering measures", note the COUNT( CASE....
SELECT
base.state as state,
COUNT( CASE WHEN base.fac_type='AIRPORT' THEN 1 END) as airports,
COUNT( CASE WHEN base.fac_type='HELIPORT' THEN 1 END) as heliports,
COUNT( 1) as total
FROM `malloy-data.faa.airports` as base
GROUP BY 1
ORDER BY 2 descThis allows for aggregating over a subset of rows for only that field.
Here's a truncated example from our readme:
func if_valid x = is_valid_price ? x : null
from prices
derive [
return_total: prices_adj | ret | if_valid
return_usd: prices_usd | ret | if_valid
]With an inline filter, this could be:
from prices
derive [
return_total: prices_adj | ret | filter is_valid_price
return_usd: prices_usd | ret | filter is_valid_price
]Or with an expression rather than a column:
from prices
derive [
return_total: prices_adj | ret | filter price > 1
return_usd: prices_usd | ret | filter price > 1
]We'd need to think through what restrictions we'd need on the filter — is any expression allowed?