Does Redshift support User-Defined Variables in SELECT?

Question

I'm reviewing some of our Redshift queries and found cases with multiple levels of nested select like the one below:

    LEFT JOIN
    (
        SELECT *
        FROM (
            SELECT
                id,
                created_at,
                min(created_at) OVER (PARTITION BY id, slug) AS transition_date
            FROM table
            WHERE status = 'cancelled'
            GROUP BY id, Y, Z, created_at
        )
        WHERE created_at = transition_date
    ) t1 ON b.id = t1.id

if this were MySQL, I would've done something like this to remove one level of nested select:

    LEFT JOIN
    (
        SELECT
            id,
            created_at,
            @tdate := min(created_at) OVER (PARTITION BY id, slug) AS transition_date
        FROM table
        WHERE status = 'cancelled' and @tdate = bul.created_at
        GROUP BY id, Y, Z, created_at
    ) t1 ON b.id = t1.id

Is it possible to so something similar in RedShift?

--- update forgot to include GROUP BY in the nested SELECT, which may affect the answer

Answer 1

You can move the condition for the transition_date into the JOIN condition:

LEFT JOIN
(
    SELECT
        id,
        created_at,
        min(created_at) OVER (PARTITION BY id, slug) AS transition_date
    FROM table
    WHERE status = 'cancelled'
) t1 ON b.id = t1.id AND t1.created_at = t1.transition_date