Description

UNNEST is a function used to expand arrays or nested data structures, 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

Parameter Description

Array Expression : The array or nested data structure (such as multidimensional arrays, structs) that needs to be expanded. Alias (Column Name): Optional, specify an alias and column name for the expanded columns. LEFT JOIN : Optional, retain all rows of the left table even if there is no matching data in the right table (UNNEST result).

** Example**

1. Basic Array Expansion Expand a one-dimensional array into multiple rows, with each row corresponding to an element in the array. Example:

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

2. Multiple Array Expansion Supports expanding multiple arrays simultaneously, aligned by rows. If the 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 multidimensional arrays (recursive expansion) to generate 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. Using with JOIN Can be associated with other tables through 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, there will be no result after expansion.
• If the array is empty ([]), there will also be no result after expansion.
  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. ：Filter Expansion Results

-- Only keep 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

Precautions

1. Parameter Type Restrictions
   UNNEST only accepts arrays or nested structures as input, passing non-array types will result in an error.

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