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].

List Operators

The following operators are supported for lists:

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

Map Functions

Union Functions

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_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.