Nested Functions

This section describes functions and operators for examining and manipulating nested values. There are three nested data types: lists, structs, and maps.

List Functions

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

Function	Description	Example	Result
list[index]	Bracket notation serves as an alias for list_extract.	l[3]	6
list_extract(list, index)	Extract the indexth (1-based) value from the list.	list_extract(l, 3)	6
list_element(list, index)	Alias for list_extract.	list_element(l, 3)	6
array_extract(list, index)	Alias for list_extract.	array_extract(l, 3)	6
list[begin:end]	Bracket notation with colon is an alias for list_slice. Missing arguments are interpreted as NULLs.	l[2:3]	[5, 6]
list_slice(list, begin, end)	Extract a sublist using slice conventions. NULLs are interpreted as the bounds of the LIST. Negative values are accepted.	list_slice(l, 2, NULL)	[5, 6]
array_slice(list, begin, end)	Alias for list_slice.	array_slice(l, 2, NULL)	[5, 6]
array_pop_front(list)	Returns the list without the first element.	array_pop_front(l)	[5, 6]
array_pop_back(list)	Returns the list without the last element.	array_pop_back(l)	[4, 5]
list_value(any, ...)	Create a LIST containing the argument values.	list_value(4, 5, 6)	[4, 5, 6]
list_pack(any, ...)	Alias for list_value.	list_pack(4, 5, 6)	[4, 5, 6]
len(list)	Return the length of the list.	len([1, 2, 3])	3
array_length(list)	Alias for len.	array_length([1, 2, 3])	3
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
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]
list_concat(list1, list2)	Concatenates two lists.	list_concat([2, 3], [4, 5, 6])	[2, 3, 4, 5, 6]
list_cat(list1, list2)	Alias for list_concat.	list_cat([2, 3], [4, 5, 6])	[2, 3, 4, 5, 6]
array_concat(list1, list2)	Alias for list_concat.	array_concat([2, 3], [4, 5, 6])	[2, 3, 4, 5, 6]
array_cat(list1, list2)	Alias for list_concat.	array_cat([2, 3], [4, 5, 6])	[2, 3, 4, 5, 6]
list_prepend(element, list)	Prepends element to list.	list_prepend(3, [4, 5, 6])	[3, 4, 5, 6]
array_prepend(element, list)	Alias for list_prepend.	array_prepend(3, [4, 5, 6])	[3, 4, 5, 6]
array_push_front(list, element)	Alias for list_prepend.	array_push_front(l, 3)	[3, 4, 5,6]
list_append(list, element)	Appends element to list.	list_append([2, 3], 4)	[2, 3, 4]
array_append(list, element)	Alias for list_append.	array_append([2, 3], 4)	[2, 3, 4]
array_push_back(list, element)	Alias for list_append.	array_push_back(l, 7)	[4, 5, 6, 7]
list_contains(list, element)	Returns true if the list contains the element.	list_contains([1, 2, NULL], 1)	true
list_has(list, element)	Alias for list_contains.	list_has([1, 2, NULL], 1)	true
array_contains(list, element)	Alias for list_contains.	array_contains([1, 2, NULL], 1)	true
array_has(list, element)	Alias for list_contains.	array_has([1, 2, NULL], 1)	true
list_intersect(list1, list2)	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]
array_intersect(list1, list2)	Alias for list_intersect.	array_intersect([1,2,3], [2,3,4])	[2, 3]
list_has_any(list1, list2)	Returns true if any elements exist is both lists.	list_has_any([1,2,3], [2,3,4])	true
array_has_any(list1, list2)	Alias for list_has_any.	array_has_any([1,2,3], [2,3,4])	true
list_has_all(list, sub-list)	Returns true if all elements of sub-list exist in list.	list_has_all(l, [4,6])	true
array_has_all(list, sub-list)	Alias for list_has_all	array_has_all(l, [4,6])	true
list_position(list, element)	Returns the index of the element if the list contains the element.	list_contains([1, 2, NULL], 2)	2
list_indexof(list, element)	Alias for list_position.	list_indexof([1, 2, NULL], 2)	2
array_position(list, element)	Alias for list_position.	array_position([1, 2, NULL], 2)	2
array_indexof(list, element)	Alias for list_position.	array_indexof([1, 2, NULL], 2)	2
list_aggregate(list, name)	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_aggr(list, name)	Alias for list_aggregate.	list_aggr([1, 2, NULL], 'min')	1
array_aggregate(list, name)	Alias for list_aggregate.	array_aggregate([1, 2, NULL], 'min')	1
array_aggr(list, name)	Alias for list_aggregate.	array_aggr([1, 2, NULL], 'min')	1
list_sort(list)	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]
array_sort(list)	Alias for list_sort.	array_sort([3, 6, 1, 2])	[1, 2, 3, 6]
list_reverse_sort(list)	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]
array_reverse_sort(list)	Alias for list_reverse_sort.	array_reverse_sort([3, 6, 1, 2])	[6, 3, 2, 1]
list_transform(list, lambda)	Returns a list that is the result of applying the lambda function to each element of the input list. See the Lambda Functions section for more details.	list_transform(l, x -> x + 1)	[5, 6, 7]
array_transform(list, lambda)	Alias for list_transform.	array_transform(l, x -> x + 1)	[5, 6, 7]
list_apply(list, lambda)	Alias for list_transform.	list_apply(l, x -> x + 1)	[5, 6, 7]
array_apply(list, lambda)	Alias for list_transform.	array_apply(l, x -> x + 1)	[5, 6, 7]
list_filter(list, lambda)	Constructs a list from those elements of the input list for which the lambda function returns true. See the Lambda Functions section for more details.	list_filter(l, x -> x > 4)	[5, 6]
array_filter(list, lambda)	Alias for list_filter.	array_filter(l, x -> x > 4)	[5, 6]
list_distinct(list)	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]
array_distinct(list)	Alias for list_distinct.	array_distinct([1, 1, NULL, -3, 1, 5])	[1, 5, -3]
list_unique(list)	Counts the unique elements of a list.	list_unique([1, 1, NULL, -3, 1, 5])	3
array_unique(list)	Alias for list_unique.	array_unique([1, 1, NULL, -3, 1, 5])	3
list_any_value(list)	Returns the first non-null value in the list	list_any_value([NULL, -3])	-3
list_resize(list, size[, value])	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]
array_resize(list, size[, value])	Alias for list_resize.	array_resize([1,2,3], 5, 0)	[1, 2, 3, 0, 0]

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]

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 serves as an alias for struct_extract.	({'i': 3, 's': 'string'}).s	string
struct[entry]	Bracket notation serves as an alias for struct_extract.	({'i': 3, 's': 'string'})['s']	string
row(any, ...)	Create a STRUCT containing the argument values. If the values are column references, the entry name will be the column name; otherwise it will be the string 'vN' where N is the (1-based) position of the argument.	row(i, i % 4, i / 4)	{'i': 3, 'v2': 3, 'v3': 0}
struct_extract(struct, 'entry')	Extract the named entry from the struct.	struct_extract(s, 'i')	4
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': 3, 's': 'string'}
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}

Map Functions

Function	Description	Example	Result
map[entry]	Alias for element_at	map([100, 5], ['a', 'b'])[100]	[a]
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_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]
cardinality(map)	Return the size of the map (or the number of entries in the map).	cardinality( map([4, 2], ['a', 'b']) );	2
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()	Returns an empty map.	map()	{}
map_keys(map)	Return a list of all keys in the map.	map_keys(map([100, 5], [42, 43]))	[100, 42]
map_values(map)	Return a list of all values in the map.	map_values(map([100, 5], [42, 43]))	[5, 33]
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]))	[{'k': 100, 'v': 42}, {'k': 5, 'v': 43}]

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 │
└─────────────────────┘

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_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 these 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

(parameter1, parameter2, ...) -> expression. If the lambda function has only one parameter, then the brackets can be omitted. The parameters can have any names.

param -> param > 1
duck -> CONTAINS(CONCAT(duck, 'DB'), 'duck')
(x, y) -> x + y

Transform

list_transform(list, lambda)

Returns a list that is the result of applying the lambda function to each element of the input list. The lambda function must have exactly one left-hand side parameter. The return type of the lambda function defines the type of the list elements.

-- incrementing each list element by one
SELECT list_transform([1, 2, NULL, 3], x -> x + 1)
----
[2, 3, NULL, 4]

-- transforming strings
SELECT list_transform(['duck', 'a', 'b'], duck -> CONCAT(duck, 'DB'))
----
[duckDB, aDB, bDB]

-- combining lambda functions with other functions
SELECT list_transform([5, NULL, 6], x -> COALESCE(x, 0) + 1)
----
[6, 1, 7]

Filter

list_filter(list, lambda)

Constructs a list from those elements of the input list for which the lambda function returns true. The lambda function must have exactly one left-hand side parameter and its return type must be of type BOOLEAN.

-- filter out negative values
SELECT list_filter([5, -6, NULL, 7], x -> x > 0)
----
[5, 7]

-- divisible by 2 and 5
SELECT list_filter(list_filter([2, 4, 3, 1, 20, 10, 3, 30], x -> x % 2 == 0), y -> y % 5 == 0)
----
[20, 10, 30]

-- in combination with range(...) to construct lists
SELECT list_filter([1, 2, 3, 4], x -> x > #1) FROM range(4)
----
[1, 2, 3, 4]
[2, 3, 4]
[3, 4]
[4]
[]

Lambda functions can be arbitrarily nested.

-- nested lambda functions to get all squares of even list elements
SELECT list_transform(list_filter([0, 1, 2, 3, 4, 5], x -> x % 2 = 0), y -> y * y)
----
[0, 4, 16]

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_subscript(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 │
└───┘

Related Functions

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