Description

UNNEST is a function used to expand arrays or nested data structures. It is commonly used to convert array-type columns into multiple rows of data for row-level analysis.

Syntax

SELECT [column_name] FROM UNNEST(array_expression) [AS alias(column_name)];
SELECT [column_name] FROM table_name, UNNEST(array_column) [AS alias(column_name)]; -- Implicit JOIN

Parameters

array_expression: The array or nested data structure to expand (such as multi-dimensional arrays, structs). alias(column_name): Optional, specifies an alias and column name for the expanded column. LEFT JOIN: Optional, retains all rows from the left table even if the right table (UNNEST result) has no matching data.

Examples

1. Basic array expansion Expands a one-dimensional array into multiple rows, with each row corresponding to one element in the array. Example:

-- Input: array(1,2,3)
SELECT * FROM UNNEST(array(1,2,3));
-- Output:
-- 1
-- 2
-- 3

2. Multi-array column expansion Supports expanding multiple arrays simultaneously, aligned by row. If array lengths are inconsistent, missing values are filled with NULL. Example:

-- Input: array(1,2,3), array('ab','cd')
SELECT * FROM UNNEST(array(1,2,3), array('ab','cd'));
-- Output:
-- 1    ab
-- 2    cd
-- 3    NULL

3. Nested array expansion Supports expanding multi-dimensional arrays (recursive expansion), producing flattened results. Example:

-- Input: array(array(1,2,3), array(4,5,6))
SELECT * FROM UNNEST(array(array(1,2,3), array(4,5,6)));
-- Output:
-- 1
-- 2
-- 3
-- 4
-- 5
-- 6

4. Combined use with JOIN Can be associated with other tables using JOIN or CROSS JOIN to expand array columns. Example:

-- Input: Table t contains column k and array column a
WITH t AS (SELECT * FROM VALUES (1, array(1,2,3)), (2, array(4,5)) AS t(k, a))
SELECT * FROM t, UNNEST(a);
-- Output:
-- 1    [1,2,3]    1
-- 1    [1,2,3]    2
-- 1    [1,2,3]    3
-- 2    [4,5]      4
-- 2    [4,5]      5

5. Handling NULL and empty arrays

• If the array is NULL, the expansion produces no results.
• If the array is empty ([]), the expansion also produces no results. Example:

-- Input: Array column contains NULL or empty values
CREATE VIEW student_score AS
SELECT id, scores FROM VALUES
(1, [80,85,87]),
(2, [77, NULL, 89]),
(3, NULL),
(4, []) AS students(id, scores);

SELECT id, scores, score FROM student_score, UNNEST(scores) AS t(score);
-- Output:
-- 1    [80,85,87]    80
-- 1    [80,85,87]    85
-- 1    [80,85,87]    87
-- 2    [77,null,89]  77
-- 2    [77,null,89]  NULL
-- 2    [77,null,89]  89

6. Filtering expanded results

-- Keep only even elements
WITH t AS (SELECT * FROM VALUES (1, array(1,2,3)), (2, array(4,5)) AS t(k, a))
SELECT * FROM t LEFT JOIN UNNEST(a) u(e) WHERE u.e % 2 = 0;
-- Output:
-- 1    [1,2,3]    2
-- 2    [4,5]      4

Notes

1. Parameter type restrictions UNNEST only accepts arrays or nested structures as input. Passing a non-array type will result in an error.

   -- Error example: Input is an integer
   SELECT * FROM UNNEST(1); -- Error: expect array type