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Nested Functions

This section describes functions and operators for examining and manipulating nested values. There are five nested data types: ARRAY, LIST, MAP, STRUCT, and UNION.

List Functions

In the descriptions, l is the three element list [4, 5, 6].

Function Aliases Description Example Result
list[index]   Bracket notation serves as an alias for list_extract. l[3] 6
list[begin:end]   Bracket notation with colon is an alias for list_slice. l[2:3] [5, 6]
list[begin:end:step]   list_slice in bracket notation with an added step feature. l[:-:2] [4, 6]
array_pop_back(list)   Returns the list without the last element. array_pop_back(l) [4, 5]
array_pop_front(list)   Returns the list without the first element. array_pop_front(l) [5, 6]
flatten(list_of_lists)   Concatenate a list of lists into a single list. This only flattens one level of the list (see examples). flatten([[1, 2], [3, 4]]) [1, 2, 3, 4]
len(list) array_length Return the length of the list. len([1, 2, 3]) 3
list_aggregate(list, name) list_aggr, aggregate, array_aggregate, array_aggr Executes the aggregate function name on the elements of list. See the List Aggregates section for more details. list_aggregate([1, 2, NULL], 'min') 1
list_any_value(list)   Returns the first non-null value in the list list_any_value([NULL, -3]) -3
list_append(list, element) array_append, array_push_back Appends element to list. list_append([2, 3], 4) [2, 3, 4]
list_concat(list1, list2) list_cat, array_concat, array_cat Concatenates two lists. list_concat([2, 3], [4, 5, 6]) [2, 3, 4, 5, 6]
list_contains(list, element) list_has, array_contains, array_has Returns true if the list contains the element. list_contains([1, 2, NULL], 1) true
list_cosine_similarity(list1, list2)   Compute the cosine similarity between two lists list_cosine_similarity([1, 2, 3], [1, 2, 5]) 0.9759000729485332
list_distance(list1, list2)   Calculates the Euclidean distance between two points with coordinates given in two inputs lists of equal length. list_distance([1, 2, 3], [1, 2, 5]) 2.0
list_distinct(list) array_distinct Removes all duplicates and NULLs from a list. Does not preserve the original order. list_distinct([1, 1, NULL, -3, 1, 5]) [1, 5, -3]
list_dot_product(list1, list2) list_inner_product Computes the dot product of two same-sized lists of numbers. list_dot_product([1, 2, 3], [1, 2, 5]) 20.0
list_extract(list, index) list_element, array_extract Extract the indexth (1-based) value from the list. list_extract(l, 3) 6
list_filter(list, lambda) array_filter, filter Constructs a list from those elements of the input list for which the lambda function returns true. See the Lambda Functions page for more details. list_filter(l, x -> x > 4) [5, 6]
list_grade_up(list) array_grade_up Works like sort, but the results are the indexes that correspond to the position in the original list instead of the actual values. list_grade_up([30, 10, 40, 20]) [2, 4, 1, 3]
list_has_all(list, sub-list) array_has_all Returns true if all elements of sub-list exist in list. list_has_all(l, [4, 6]) true
list_has_any(list1, list2) array_has_any Returns true if any elements exist is both lists. list_has_any([1, 2, 3], [2, 3, 4]) true
list_intersect(list1, list2) array_intersect Returns a list of all the elements that exist in both l1 and l2, without duplicates. list_intersect([1, 2, 3], [2, 3, 4]) [2, 3]
list_position(list, element) list_indexof, array_position, array_indexof Returns the index of the element if the list contains the element. list_contains([1, 2, NULL], 2) 2
list_prepend(element, list) array_prepend, array_push_front Prepends element to list. list_prepend(3, [4, 5, 6]) [3, 4, 5, 6]
list_reduce(list, lambda) array_reduce, reduce Returns a single value that is the result of applying the lambda function to each element of the input list. See the Lambda Functions page for more details. list_reduce(l, (x, y) -> x + y) 15
list_resize(list, size[, value]) array_resize Resizes the list to contain size elements. Initializes new elements with value or NULL if value is not set. list_resize([1, 2, 3], 5, 0) [1, 2, 3, 0, 0]
list_reverse_sort(list) array_reverse_sort Sorts the elements of the list in reverse order. See the Sorting Lists section for more details about the null sorting order. list_reverse_sort([3, 6, 1, 2]) [6, 3, 2, 1]
list_reverse(list) array_reverse Reverses the list. list_reverse(l) [6, 5, 4]
list_select(value_list, index_list) array_select Returns a list based on the elements selected by the index_list. list_select([10, 20, 30, 40], [1, 4]) [10, 40]
list_slice(list, begin, end, step) array_slice list_slice with added step feature. list_slice(l, 1, 3, 2) [4, 6]
list_slice(list, begin, end) array_slice Extract a sublist using slice conventions. Negative values are accepted. See slicing. list_slice(l, 2, 3) [5, 6]
list_sort(list) array_sort Sorts the elements of the list. See the Sorting Lists section for more details about the sorting order and the null sorting order. list_sort([3, 6, 1, 2]) [1, 2, 3, 6]
list_transform(list, lambda) array_transform, apply, list_apply, array_apply Returns a list that is the result of applying the lambda function to each element of the input list. See the Lambda Functions page for more details. list_transform(l, x -> x + 1) [5, 6, 7]
list_unique(list) array_unique Counts the unique elements of a list. list_unique([1, 1, NULL, -3, 1, 5]) 3
list_value(any, ...) list_pack Create a LIST containing the argument values. list_value(4, 5, 6) [4, 5, 6]
list_where(value_list, mask_list) array_where Returns a list with the BOOLEANs in mask_list applied as a mask to the value_list. list_where([10, 20, 30, 40], [true, false, false, true]) [10, 40]
list_zip(list1, list2, ...) array_zip Zips k LISTs to a new LIST whose length will be that of the longest list. Its elements are structs of k elements list_1, …, list_k. Elements missing will be replaced with NULL. list_zip([1, 2], [3, 4], [5, 6]) [{'list_1': 1, 'list_2': 3, 'list_3': 5}, {'list_1': 2, 'list_2': 4, 'list_3': 6}]
unnest(list)   Unnests a list by one level. Note that this is a special function that alters the cardinality of the result. See the unnest page for more details. unnest([1, 2, 3]) 1, 2, 3

List Operators

The following operators are supported for lists:

Operator Description Example Result
&& Alias for list_intersect [1, 2, 3, 4, 5] && [2, 5, 5, 6] [2, 5]
@> Alias for list_has_all, where the list on the right of the operator is the sublist. [1, 2, 3, 4] @> [3, 4, 3] true
<@ Alias for list_has_all, where the list on the left of the operator is the sublist. [1, 4] <@ [1, 2, 3, 4] true
|| Alias for list_concat [1, 2, 3] || [4, 5, 6] [1, 2, 3, 4, 5, 6]
<=> Alias for list_cosine_similarity [1, 2, 3] <=> [1, 2, 5] 0.9759000729485332
<-> Alias for list_distance [1, 2, 3] <-> [1, 2, 5] 2.0

List Comprehension

Python-style list comprehension can be used to compute expressions over elements in a list. For example:

SELECT [lower(x) FOR x IN strings]
FROM (VALUES (['Hello', '', 'World'])) t(strings);
-- [hello, , world]
SELECT [upper(x) FOR x IN strings IF len(x) > 0]
FROM (VALUES (['Hello', '', 'World'])) t(strings);
-- [HELLO, WORLD]

Struct Functions

Function Description Example Result
struct.entry Dot notation that serves as an alias for struct_extract from named STRUCTs. ({'i': 3, 's': 'string'}).i 3
struct[entry] Bracket notation that serves as an alias for struct_extract from named STRUCTs. ({'i': 3, 's': 'string'})['i'] 3
struct[idx] Bracket notation that serves as an alias for struct_extract from unnamed STRUCTs (tuples), using an index (1-based). (row(42, 84))[1] 42
row(any, ...) Create an unnamed STRUCT (tuple) containing the argument values. row(i, i % 4, i / 4) (10, 2, 2.5)
struct_extract(struct, 'entry') Extract the named entry from the STRUCT. struct_extract({'i': 3, 'v2': 3, 'v3': 0}, 'i') 3
struct_extract(struct, idx) Extract the entry from an unnamed STRUCT (tuple) using an index (1-based). struct_extract(row(42, 84), 1) 42
struct_insert(struct, name := any, ...) Add field(s)/value(s) to an existing STRUCT with the argument values. The entry name(s) will be the bound variable name(s). struct_insert({'a': 1}, b := 2) {'a': 1, 'b': 2}
struct_pack(name := any, ...) Create a STRUCT containing the argument values. The entry name will be the bound variable name. struct_pack(i := 4, s := 'string') {'i': 4, 's': string}

Map Functions

Function Description Example Result
cardinality(map) Return the size of the map (or the number of entries in the map). cardinality(map([4, 2], ['a', 'b'])) 2
element_at(map, key) Return a list containing the value for a given key or an empty list if the key is not contained in the map. The type of the key provided in the second parameter must match the type of the map’s keys else an error is returned. element_at(map([100, 5], [42, 43]), 100) [42]
map_entries(map) Return a list of struct(k, v) for each key-value pair in the map. map_entries(map([100, 5], [42, 43])) [{'key': 100, 'value': 42}, {'key': 5, 'value': 43}]
map_extract(map, key) Alias of element_at. Return a list containing the value for a given key or an empty list if the key is not contained in the map. The type of the key provided in the second parameter must match the type of the map’s keys else an error is returned. map_extract(map([100, 5], [42, 43]), 100) [42]
map_from_entries(STRUCT(k, v)[]) Returns a map created from the entries of the array map_from_entries([{k: 5, v: 'val1'}, {k: 3, v: 'val2'}]) {5=val1, 3=val2}
map_keys(map) Return a list of all keys in the map. map_keys(map([100, 5], [42,43])) [100, 5]
map_values(map) Return a list of all values in the map. map_values(map([100, 5], [42, 43])) [42, 43]
map() Returns an empty map. map() {}
map[entry] Alias for element_at map([100, 5], ['a', 'b'])[100] [a]

Union Functions

Function Description Example Result
union.tag Dot notation serves as an alias for union_extract. (union_value(k := 'hello')).k string
union_extract(union, 'tag') Extract the value with the named tags from the union. NULL if the tag is not currently selected union_extract(s, 'k') hello
union_value(tag := any) Create a single member UNION containing the argument value. The tag of the value will be the bound variable name. union_value(k := 'hello') 'hello'::UNION(k VARCHAR)
union_tag(union) Retrieve the currently selected tag of the union as an Enum. union_tag(union_value(k := 'foo')) 'k'

Range Functions

The functions range and generate_series create a list of values in the range between start and stop. The start parameter is inclusive. For the range function, the stop parameter is exclusive, while for generate_series, it is inclusive.

Based on the number of arguments, the following variants exist:

  • range(start, stop, step)
  • range(start, stop)
  • range(stop)
  • generate_series(start, stop, step)
  • generate_series(start, stop)
  • generate_series(stop)

The default value of start is 0 and the default value of step is 1.

SELECT range(5);
-- [0, 1, 2, 3, 4]
SELECT range(2, 5);
-- [2, 3, 4]
SELECT range(2, 5, 3);
-- [2]
SELECT generate_series(5);
-- [0, 1, 2, 3, 4, 5]
SELECT generate_series(2, 5);
-- [2, 3, 4, 5]
SELECT generate_series(2, 5, 3);
-- [2, 5]

Date ranges are also supported:

SELECT *
FROM range(DATE '1992-01-01', DATE '1992-03-01', INTERVAL '1' MONTH);
┌─────────────────────┐
│        range        │
├─────────────────────┤
│ 1992-01-01 00:00:00 │
│ 1992-02-01 00:00:00 │
└─────────────────────┘

Slicing

The function list_slice can be used to extract a sublist from a list. The following variants exist:

  • list_slice(list, begin, end)
  • list_slice(list, begin, end)
  • array_slice(list, begin, end, step)
  • array_slice(list, begin, end, step)
  • list[begin:end]
  • list[begin:end:step]

list

  • Is the list to be sliced

begin

  • Is the index of the first element to be included in the slice
  • When begin < 0 the index is counted from the end of the list
  • When begin < 0 and -begin > length, begin is clamped to the beginning of the list
  • When begin > length, the result is an empty list
  • Bracket Notation: When begin is omitted, it defaults to the beginning of the list

end

  • Is the index of the last element to be included in the slice
  • When end < 0 the index is counted from the end of the list
  • When end > length, end is clamped to length
  • When end < begin, the result is an empty list
  • Bracket Notation: When end is omitted, it defaults to the end of the list. When end is omitted and a step is provided, end must be replaced with a -

step (optional)

  • Is the step size between elements in the slice
  • When step < 0 the slice is reversed, and begin and end are swapped
  • Must be non-zero
SELECT list_slice([1, 2, 3, 4, 5], 2, 4);
-- [2, 3, 4]
SELECT ([1, 2, 3, 4, 5])[2:4:2];
-- [2, 4]
SELECT([1, 2, 3, 4, 5])[4:2:-2];
-- [4, 2]
SELECT ([1, 2, 3, 4, 5])[:];
-- [1, 2, 3, 4, 5]
SELECT ([1, 2, 3, 4, 5])[:-:2];
-- [1, 3, 5]
SELECT ([1, 2, 3, 4, 5])[:-:-2];
-- [5, 3, 1]

List Aggregates

The function list_aggregate allows the execution of arbitrary existing aggregate functions on the elements of a list. Its first argument is the list (column), its second argument is the aggregate function name, e.g., min, histogram or sum.

list_aggregate accepts additional arguments after the aggregate function name. These extra arguments are passed directly to the aggregate function, which serves as the second argument of list_aggregate.

SELECT list_aggregate([1, 2, -4, NULL], 'min');
-- -4
SELECT list_aggregate([2, 4, 8, 42], 'sum');
-- 56
SELECT list_aggregate([[1, 2], [NULL], [2, 10, 3]], 'last');
-- [2, 10, 3]
SELECT list_aggregate([2, 4, 8, 42], 'string_agg', '|');
-- 2|4|8|42

The following is a list of existing rewrites. Rewrites simplify the use of the list aggregate function by only taking the list (column) as their argument. list_avg, list_var_samp, list_var_pop, list_stddev_pop, list_stddev_samp, list_sem, list_approx_count_distinct, list_bit_xor, list_bit_or, list_bit_and, list_bool_and, list_bool_or, list_count, list_entropy, list_last, list_first, list_kurtosis, list_kurtosis_pop, list_min, list_max, list_product, list_skewness, list_sum, list_string_agg, list_mode, list_median, list_mad and list_histogram.

SELECT list_min([1, 2, -4, NULL]);
-- -4
SELECT list_sum([2, 4, 8, 42]);
-- 56
SELECT list_last([[1, 2], [NULL], [2, 10, 3]]);
-- [2, 10, 3]

array_to_string

Concatenates list/array elements using an optional delimiter.

SELECT array_to_string([1, 2, 3], '-') AS str;
-- 1-2-3
-- this is equivalent to the following SQL
SELECT list_aggr([1, 2, 3], 'string_agg', '-') AS str;
-- 1-2-3

Sorting Lists

The function list_sort sorts the elements of a list either in ascending or descending order. In addition, it allows to provide whether NULL values should be moved to the beginning or to the end of the list.

By default if no modifiers are provided, DuckDB sorts ASC NULLS FIRST, i.e., the values are sorted in ascending order and NULL values are placed first. This is identical to the default sort order of SQLite. The default sort order can be changed using PRAGMA statements.

list_sort leaves it open to the user whether they want to use the default sort order or a custom order. list_sort takes up to two additional optional parameters. The second parameter provides the sort order and can be either ASC or DESC. The third parameter provides the NULL sort order and can be either NULLS FIRST or NULLS LAST.

-- default sort order and default NULL sort order
SELECT list_sort([1, 3, NULL, 5, NULL, -5]);
----
[NULL, NULL, -5, 1, 3, 5]
-- only providing the sort order
SELECT list_sort([1, 3, NULL, 2], 'ASC');
----
[NULL, 1, 2, 3]
-- providing the sort order and the NULL sort order
SELECT list_sort([1, 3, NULL, 2], 'DESC', 'NULLS FIRST');
----
[NULL, 3, 2, 1]

list_reverse_sort has an optional second parameter providing the NULL sort order. It can be either NULLS FIRST or NULLS LAST.

-- default NULL sort order
SELECT list_sort([1, 3, NULL, 5, NULL, -5]);
----
[NULL, NULL, -5, 1, 3, 5]
-- providing the NULL sort order
SELECT list_reverse_sort([1, 3, NULL, 2], 'NULLS LAST');
----
[3, 2, 1, NULL]

Lambda Functions

DuckDB supports lambda functions in the form (parameter1, parameter2, ...) -> expression. For details, see the lambda functions page.

Flatten

The flatten function is a scalar function that converts a list of lists into a single list by concatenating each sub-list together. Note that this only flattens one level at a time, not all levels of sub-lists.

-- Convert a list of lists into a single list
SELECT 
    flatten([
        [1, 2],
        [3, 4]
    ]);
----
[1, 2, 3, 4]
-- If the list has multiple levels of lists, 
-- only the first level of sub-lists is concatenated into a single list
SELECT 
    flatten([
        [
            [1, 2],
            [3, 4],
        ],
        [
            [5, 6],
            [7, 8],
        ]
    ]);
----
[[1, 2], [3, 4], [5, 6], [7, 8]] 

In general, the input to the flatten function should be a list of lists (not a single level list). However, the behavior of the flatten function has specific behavior when handling empty lists and NULL values.

-- If the input list is empty, return an empty list
SELECT flatten([]);
----
[]
-- If the entire input to flatten is NULL, return NULL
SELECT flatten(NULL);
----
NULL
-- If a list whose only entry is NULL is flattened, return an empty list
SELECT flatten([NULL]);
----
[]
-- If the sub-list in a list of lists only contains NULL, 
-- do not modify the sub-list
-- (Note the extra set of parentheses vs. the prior example)
SELECT flatten([[NULL]]);
----
[NULL]
-- Even if the only contents of each sub-list is NULL,
-- still concatenate them together
-- Note that no de-duplication occurs when flattening. 
-- See list_distinct function for de-duplication.
SELECT flatten([[NULL],[NULL]]);
----
[NULL, NULL]

generate_subscripts

The generate_subscripts(arr, dim) function generates indexes along the dimth dimension of array arr.

SELECT generate_subscripts([4, 5, 6], 1) AS i;
┌───┐
│ i │
├───┤
│ 1 │
│ 2 │
│ 3 │
└───┘

There are also aggregate functions list and histogram that produces lists and lists of structs. The unnest function is used to unnest a list by one level.

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Last modified: 2024-03-02