NOT EXISTS together with SELECT NULL

Question

I'm trying to figure out what this SQL query is doing, more specifically in the part that starts after NOT EXISTS:

SELECT
    order_num,
    MIN(order_date)
FROM
    orders
WHERE
    order_date >= '01.01.2019'
    AND
    NOT EXISTS
    (
        SELECT
            NULL
        FROM
            result
        WHERE
            unique_id = '201895'
            AND
            result = order_num
    )
GROUP BY
    order_num

Answer 1

SELECT NULL still returns rows from the query. It does the same as if it was:

[...] EXISTS( SELECT 1 FROM [...]

or

[...] EXISTS( SELECT Id FROM [...]

It's just one way to make it clear that the value is not used.