Presto 0.186 Documentation

6.17. Lambda Expressions and Functions

6.17. Lambda Expressions and Functions

Lambda Expression

Lambda expressions are written with ->:

x -> x + 1
(x, y) -> x + y
x -> regexp_like(x, 'a+')
x -> x[1] / x[2]
x -> IF(x > 0, x, -x)
x -> COALESCE(x, 0)
x -> CAST(x AS JSON)
x -> x + TRY(1 / 0)

Most SQL expressions can be used in a lambda body, with a few exceptions:

  • Subqueries are not supported. x -> 2 + (SELECT 3)
  • Aggregations are not supported. x -> max(y)

Lambda Functions

filter(array<T>, function<T, boolean>) → ARRAY<T>

Constructs an array from those elements of array for which function returns true:

SELECT filter(ARRAY [], x -> true); -- []
SELECT filter(ARRAY [5, -6, NULL, 7], x -> x > 0); -- [5, 7]
SELECT filter(ARRAY [5, NULL, 7, NULL], x -> x IS NOT NULL); -- [5, 7]
map_filter(map<K, V>, function<K, V, boolean>) → MAP<K,V>

Constructs a map from those entries of map for which function returns true:

SELECT map_filter(MAP(ARRAY[], ARRAY[]), (k, v) -> true); -- {}
SELECT map_filter(MAP(ARRAY[10, 20, 30], ARRAY['a', NULL, 'c']), (k, v) -> v IS NOT NULL); -- {10 -> a, 30 -> c}
SELECT map_filter(MAP(ARRAY['k1', 'k2', 'k3'], ARRAY[20, 3, 15]), (k, v) -> v > 10); -- {k1 -> 20, k3 -> 15}
reduce(array<T>, initialState S, inputFunction<S, T, S>, outputFunction<S, R>) → R

Returns a single value reduced from array. inputFunction will be invoked for each element in array in order. In addition to taking the element, inputFunction takes the current state, initially initialState, and returns the new state. outputFunction will be invoked to turn the final state into the result value. It may be identity function (i -> i). For example:

SELECT reduce(ARRAY [], 0, (s, x) -> s + x, s -> s); -- 0
SELECT reduce(ARRAY [5, 20, 50], 0, (s, x) -> s + x, s -> s); -- 75
SELECT reduce(ARRAY [5, 20, NULL, 50], 0, (s, x) -> s + x, s -> s); -- NULL
SELECT reduce(ARRAY [5, 20, NULL, 50], 0, (s, x) -> s + COALESCE(x, 0), s -> s); -- 75
SELECT reduce(ARRAY [5, 20, NULL, 50], 0, (s, x) -> IF(x IS NULL, s, s + x), s -> s); -- 75
SELECT reduce(ARRAY [2147483647, 1], CAST (0 AS BIGINT), (s, x) -> s + x, s -> s); -- 2147483648
SELECT reduce(ARRAY [5, 6, 10, 20], -- calculates arithmetic average: 10.25
              CAST(ROW(0.0, 0) AS ROW(sum DOUBLE, count INTEGER)),
              (s, x) -> CAST(ROW(x + s.sum, s.count + 1) AS ROW(sum DOUBLE, count INTEGER)),
              s -> IF(s.count = 0, NULL, s.sum / s.count));
transform(array<T>, function<T, U>) → ARRAY<U>

Returns an array that applies function to each element of array:

SELECT transform(ARRAY [], x -> x + 1); -- []
SELECT transform(ARRAY [5, 6], x -> x + 1); -- [6, 7]
SELECT transform(ARRAY [5, NULL, 6], x -> COALESCE(x, 0) + 1); -- [6, 1, 7]
SELECT transform(ARRAY ['x', 'abc', 'z'], x -> x || '0'); -- ['x0', 'abc0', 'z0']
SELECT transform(ARRAY [ARRAY [1, NULL, 2], ARRAY[3, NULL]], a -> filter(a, x -> x IS NOT NULL)); -- [[1, 2], [3]]
transform_keys(map<K1, V>, function<K1, V, K2>) → MAP<K2,V>

Returns a map that applies function to each entry of map and transforms the keys:

SELECT transform_keys(MAP(ARRAY[], ARRAY[]), (k, v) -> k + 1); -- {}
SELECT transform_keys(MAP(ARRAY [1, 2, 3], ARRAY ['a', 'b', 'c']), (k, v) -> k + 1); -- {2 -> a, 3 -> b, 4 -> c}
SELECT transform_keys(MAP(ARRAY ['a', 'b', 'c'], ARRAY [1, 2, 3]), (k, v) -> v * v); -- {1 -> 1, 4 -> 2, 9 -> 3}
SELECT transform_keys(MAP(ARRAY ['a', 'b'], ARRAY [1, 2]), (k, v) -> k || CAST(v as VARCHAR)); -- {a1 -> 1, b2 -> 2}
SELECT transform_keys(MAP(ARRAY [1, 2], ARRAY [1.0, 1.4]), -- {one -> 1.0, two -> 1.4}
                      (k, v) -> MAP(ARRAY[1, 2], ARRAY['one', 'two'])[k]);
transform_values(map<K, V1>, function<K, V1, V2>) → MAP<K, V2>

Returns a map that applies function to each entry of map and transforms the values:

SELECT transform_values(MAP(ARRAY[], ARRAY[]), (k, v) -> v + 1); -- {}
SELECT transform_values(MAP(ARRAY [1, 2, 3], ARRAY [10, 20, 30]), (k, v) -> v + 1); -- {1 -> 11, 2 -> 22, 3 -> 33}
SELECT transform_values(MAP(ARRAY [1, 2, 3], ARRAY ['a', 'b', 'c']), (k, v) -> k * k); -- {1 -> 1, 2 -> 4, 3 -> 9}
SELECT transform_values(MAP(ARRAY ['a', 'b'], ARRAY [1, 2]), (k, v) -> k || CAST(v as VARCHAR)); -- {a -> a1, b -> b2}
SELECT transform_values(MAP(ARRAY [1, 2], ARRAY [1.0, 1.4]), -- {1 -> one_1.0, 2 -> two_1.4}
                        (k, v) -> MAP(ARRAY[1, 2], ARRAY['one', 'two'])[k] || '_' || CAST(v AS VARCHAR));
zip_with(array<T>, array<U>, function<T, U, R>) → array<R>

Merges the two given arrays, element-wise, into a single array using function. Both arrays must have the same length:

SELECT zip_with(ARRAY[1, 3, 5], ARRAY['a', 'b', 'c'], (x, y) -> (y, x)); -- [ROW('a', 1), ROW('b', 3), ROW('c', 5)]
SELECT zip_with(ARRAY[1, 2], ARRAY[3, 4], (x, y) -> x + y); -- [4, 6]
SELECT zip_with(ARRAY['a', 'b', 'c'], ARRAY['d', 'e', 'f'], (x, y) -> concat(x, y)); -- ['ad', 'be', 'cf']