Combining Lambda Functions

Lambda functions can be nested. You can filter a list first, then transform or reduce the result.

For example, to sum only the prices above 10:

SELECT customer,
       list_reduce(
         list_filter(prices, lambda p : p > 10),
         lambda a, b : a + b
       ) AS expensive_total
FROM orders

The inner `list_filter` keeps prices above 10, then `list_reduce` sums the survivors. If `list_filter` returns an empty list, `list_reduce` returns `NULL`.

You can also transform first and filter second:

SELECT list_filter(
         list_transform([1, 2, 3, 4, 5], lambda x : x * x),
         lambda sq : sq > 10
       )
-- [16, 25]

Nesting gives you a mini data pipeline inside a single `SELECT` expression.