How do I map an array containing objects to an array containing one specific value from each object?

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Question:
How do I map an array containing objects to an array containing one specific value from each object?

Example:
Input: [{a: 1}, {a: 2}, {a: 3, b: “irrelevant”}]
Output: [1,2,3]

Answer:

select
    ARRAY_AGG(_unnest.a) as _array
from
    unnest(
        [{'a': 1}, {'a': 2}, {'a': 3, 'b': 'irrelevant'}]
    ) _unnest

Output: [1,2,3]
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