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Filtering arrays

Create an array from a collection of rows if they match the filter criteria.

WITH
dataset AS (
  SELECT ARRAY[1,2,3,4,5] AS items
)
SELECT array_agg(i) AS array_items
FROM dataset
CROSS JOIN UNNEST(items) AS t(i)
WHERE i > 3

This query returns:

+-------------+
| array_items |
+-------------+
| [4, 5]      |
+-------------+

Filter an array based on whether one of its elements contain a specific value, such as 2, as in this example:

WITH
dataset AS (
  SELECT ARRAY
  [
    ARRAY[1,2,3,4],
    ARRAY[5,6,7,8],
    ARRAY[9,0]
  ] AS items
)
SELECT i AS array_items FROM dataset
CROSS JOIN UNNEST(items) AS t(i)
WHERE contains(i, 2)

This query returns:

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

The filter function

 filter(ARRAY [list_of_values], boolean_function)

You can use the filter function on an ARRAY expression to create a new array that is the subset of the items in the list_of_values for which boolean_function is true. The filter function can be useful in cases in which you cannot use the UNNEST function.

The following example filters for values greater than zero in the array [1,0,5,-1].

SELECT filter(ARRAY [1,0,5,-1], x -> x>0)

Results

[1,5]

The following example filters for the non-null values in the array [-1, NULL, 10, NULL].

SELECT filter(ARRAY [-1, NULL, 10, NULL], q -> q IS NOT NULL)

Results

[-1,10]