Flattening nested arrays

When working with nested arrays, you often need to expand nested array elements into a single array, or expand the array into multiple rows.

Examples

To flatten a nested array's elements into a single array of values, use the flatten function. This query returns a row for each element in the array.

SELECT flatten(ARRAY[ ARRAY[1,2], ARRAY[3,4] ]) AS items

This query returns:

+-----------+
| items     |
+-----------+
| [1,2,3,4] |
+-----------+

To flatten an array into multiple rows, use CROSS JOIN in conjunction with the UNNEST operator, as in this example:

WITH dataset AS (
  SELECT
    'engineering' as department,
    ARRAY['Sharon', 'John', 'Bob', 'Sally'] as users
)
SELECT department, names FROM dataset
CROSS JOIN UNNEST(users) as t(names)

This query returns:

+----------------------+
| department  | names  |
+----------------------+
| engineering | Sharon |
+----------------------|
| engineering | John   |
+----------------------|
| engineering | Bob    |
+----------------------|
| engineering | Sally  |
+----------------------+

To flatten an array of key-value pairs, transpose selected keys into columns, as in this example:

WITH
dataset AS (
  SELECT
    'engineering' as department,
     ARRAY[
      MAP(ARRAY['first', 'last', 'age'],ARRAY['Bob', 'Smith', '40']),
      MAP(ARRAY['first', 'last', 'age'],ARRAY['Jane', 'Doe', '30']),
      MAP(ARRAY['first', 'last', 'age'],ARRAY['Billy', 'Smith', '8'])
     ] AS people
  )
SELECT names['first'] AS
 first_name,
 names['last'] AS last_name,
 department FROM dataset
CROSS JOIN UNNEST(people) AS t(names)

This query returns:

+--------------------------------------+
| first_name | last_name | department  |
+--------------------------------------+
| Bob        | Smith     | engineering |
| Jane       | Doe       | engineering |
| Billy      | Smith     | engineering |
+--------------------------------------+

From a list of employees, select the employee with the highest combined scores. UNNEST can be used in the FROM clause without a preceding CROSS JOIN as it is the default join operator and therefore implied.

WITH
dataset AS (
  SELECT ARRAY[
    CAST(ROW('Sally', 'engineering', ARRAY[1,2,3,4]) AS ROW(name VARCHAR, department VARCHAR, scores ARRAY(INTEGER))),
    CAST(ROW('John', 'finance', ARRAY[7,8,9]) AS ROW(name VARCHAR, department VARCHAR, scores ARRAY(INTEGER))),
    CAST(ROW('Amy', 'devops', ARRAY[12,13,14,15]) AS ROW(name VARCHAR, department VARCHAR, scores ARRAY(INTEGER)))
  ] AS users
),
users AS (
 SELECT person, score
 FROM
   dataset,
   UNNEST(dataset.users) AS t(person),
   UNNEST(person.scores) AS t(score)
)
SELECT person.name, person.department, SUM(score) AS total_score FROM users
GROUP BY (person.name, person.department)
ORDER BY (total_score) DESC
LIMIT 1

This query returns:

+---------------------------------+
| name | department | total_score |
+---------------------------------+
| Amy  | devops     | 54          |
+---------------------------------+

From a list of employees, select the employee with the highest individual score.

WITH
dataset AS (
 SELECT ARRAY[
   CAST(ROW('Sally', 'engineering', ARRAY[1,2,3,4]) AS ROW(name VARCHAR, department VARCHAR, scores ARRAY(INTEGER))),
   CAST(ROW('John', 'finance', ARRAY[7,8,9]) AS ROW(name VARCHAR, department VARCHAR, scores ARRAY(INTEGER))),
   CAST(ROW('Amy', 'devops', ARRAY[12,13,14,15]) AS ROW(name VARCHAR, department VARCHAR, scores ARRAY(INTEGER)))
 ] AS users
),
users AS (
 SELECT person, score
 FROM
   dataset,
   UNNEST(dataset.users) AS t(person),
   UNNEST(person.scores) AS t(score)
)
SELECT person.name, score FROM users
ORDER BY (score) DESC
LIMIT 1

This query returns:

+--------------+
| name | score |
+--------------+
| Amy  | 15    |
+--------------+

Considerations and limitations

• CROSS JOIN does not generate a Cartesian product if the main (FROM) table has an empty array or NULL on the specified column.

• Currently, UNNEST can be used only with CROSS JOIN and does not support other JOIN types (for example, LEFT JOIN UNNEST).