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List Data Type

A LIST column can have values with different lengths, but they must all have the same underlying type. LISTs are typically used to store arrays of numbers, but can contain any uniform data type, including other LISTs and STRUCTs.

LISTs are similar to Postgres’s ARRAY type. DuckDB uses the LIST terminology, but some array functions are provided for Postgres compatibility.

See the data types overview for a comparison between nested data types.

Lists can be created using the LIST_VALUE(expr, ...) function or the equivalent bracket notation [expr, ...]. The expressions can be constants or arbitrary expressions.

Creating Lists

-- List of integers
SELECT [1, 2, 3];
-- List of strings with a NULL value
SELECT ['duck', 'goose', NULL, 'heron'];
-- List of lists with NULL values
SELECT [['duck', 'goose', 'heron'], NULL, ['frog', 'toad'], []];
-- Create a list with the list_value function
SELECT list_value(1, 2, 3);
-- Create a table with an integer list column and a varchar list column
CREATE TABLE list_table (int_list INT[], varchar_list VARCHAR[]);

Retrieving from Lists

Retrieving one or more values from a list can be accomplished using brackets and slicing notation, or through list functions like list_extract. Multiple equivalent functions are provided as aliases for compatibility with systems that refer to lists as arrays. For example, the function array_slice.

-- Retrieve an element from a list using brackets. This returns 'c'
-- Note that we wrap the list creation in parenthesis so that it happens first.
-- This is only needed in our basic examples here, not when working with a list column
-- For example, this can't be parsed: SELECT ['a','b','c'][1]
SELECT (['a','b','c'])[2];
-- Use a negative index to grab the nth element from the end of the list. This returns 'c'
SELECT (['a','b','c'])[-1];
-- Any expression that evaluates to an integer can be used to retrieve a list value
-- This includes using a column to determine which index to retrieve
-- This returns 'c'
SELECT (['a','b','c'])[1 + 1];
-- The list_extract function may also be used in place of brackets for selecting individual elements. 
-- This returns c
SELECT list_extract(['a','b','c'], 2);
-- Retrieve multiple list values using a bracketed slice syntax. This returns ['b','c']
SELECT (['a','b','c'])[1:3];
-- Single sided slices are also supported. Here, grab the first 2 elements. This returns ['a','b']
SELECT (['a','b','c'])[:2];
-- Use a negative index to grab the last 2 elements. This returns ['b','c']
SELECT (['a','b','c'])[-2:];
-- The list_slice function syntax is also supported. This returns ['b','c']
SELECT list_slice(['a','b','c'],1,3);

Ordering

The ordering is defined positionally. NULL values compare greater than all other values and are considered equal to each other.

Null Comparisons

At the top level, NULL nested values obey standard SQL NULL comparison rules: comparing a NULL nested value to a non-NULL nested value produces a NULL result. Comparing nested value members , however, uses the internal nested value rules for NULLs, and a NULL nested value member will compare above a non-NULL nested value member.

Functions

See Nested Functions.

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