Redshift 是否支持 SELECT 中的用户定义变量？

Question

我正在查看我们的一些 Redshift 查询，发现具有多个嵌套选择级别的案例，如下所示：

    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

如果这是 MySQL，我会做这样的事情来删除一级嵌套选择：

    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

在 RedShift 中是否可以做类似的事情？

--- 更新 忘记在嵌套的 SELECT 中包含 GROUP BY，这可能会影响答案

Answer 1

您可以将 transition_date 的条件移动到 JOIN 条件中：

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