MySQL: 引用子查询列

Question

我目前正在为需要显示订单延迟时间的电子商务系统编写一些报告。我有一份订单的预计交货日期日志，并且能够获得初始日期和最后日期，但是做更复杂的事情（例如比较预计日期）证明是有问题的。

我的查询（简化版）如下：

SELECT orders.order_id,
       orders.date_dispatched AS actual_dispatch_date,
       (
           SELECT projected_date 
           FROM order_delivery_projections
           WHERE order_id = orders.order_id
           ORDER BY order_delivery_projection_id ASC
           LIMIT 1
       ) AS initial_delivery_projection,
       (
           SELECT projected_date 
           FROM order_delivery_projections
           WHERE order_id = orders.order_id
           ORDER BY order_delivery_projection_id DESC
           LIMIT 1
       ) AS final_delivery_projection
FROM orders
-- and some other joins here for additional report data

（仅供参考，我在这里按 ID 而不是日期排序，因为发货预测可以向前也可以向后移动，例如，如果库存发货提前到达）。

这对于提取有关订单预计交货历史的一些原始数据来说很好，但我想对数据做一些额外的分析，例如初始预计发货日期、最终预计装运日期和实际装运日期。这就是我运行遇到麻烦的地方。

我尝试在我的 SELECT 子句中添加一个 DATEDIFF(final_delivery_projection, initial_delivery_projection) 列，以查看给定的交付预测已经推迟了多少天，但 MySQL 没有。

SELECT orders.order_id,
       orders.date_dispatched AS actual_dispatch_date,
       (
           SELECT projected_date 
           FROM order_delivery_projections
           WHERE order_id = orders.order_id
           ORDER BY order_delivery_projection_id ASC
           LIMIT 1
       ) AS initial_delivery_projection,
       (
           SELECT projected_date 
           FROM order_delivery_projections
           WHERE order_id = orders.order_id
           ORDER BY order_delivery_projection_id DESC
           LIMIT 1
       ) AS final_delivery_projection,
       DATEDIFF(final_delivery_projection - initial_delivery_projection) AS projection_days_revised
FROM orders
-- and some other joins here for additional report data

Unknown column final_delivery_projection in field list

如果别名在同一语句中引用子select，则您不能在select 语句中使用别名。

我还希望能够根据预测结果排除 WHERE 子句中的订单。例如，我想排除所有最终预计发货日期早于初始预计发货日期的订单，因为我只对被搁置的订单感兴趣，而不是那些提前发货的订单。

是否可以在同一条语句中提取和处理我尝试获取的数据，或者我是否必须在客户端中进行一些 post 处理才能解决此类问题？如果在SQL可以，那怎么办呢？

Answer 1

我相信 select 的值是同时生成的。因此可能无法计算日期差异，因为当时可能不知道别名值。如果重构 Select，您可能会得到想要的结果。此外，这将使您能够访问额外的 table 值，您可能希望对子 queries/data 集进行额外分析。

SELECT orders.order_id,
       orders.date_dispatched AS actual_dispatch_date,
       First.Projected_Date AS initial_delivery_projection,
       Current.Projected_Data AS final_delivery_projection,
       dateDiff(Current.Projected_date,First.Projected_date)
FROM orders O
LEFT JOIN (SELECT projected_date, Order_ID 
           FROM order_delivery_projections
           ORDER BY order_delivery_projection_id ASC
           LIMIT 1) First
    ON First.order_id = O.order_id
LEFT JOIN (SELECT projected_date, Order_ID
           FROM order_delivery_projections
           ORDER BY order_delivery_projection_id DESC
           LIMIT 1)
    ON Current.Order_ID = O.Order_ID

是否有可能 order_Delivery_projections 中不存在订单中的 Order_ID？如果是这样，那么空值上的日期差异也可能导致错误...

Answer 2

环顾四周，您需要使用派生表才能访问 select 查询中的别名。从更多信息中查看 here

试试这个查询：

SELECT orders.order_id,
       orders.date_dispatched AS actual_dispatch_date,
       initial_delivery_projection,
       final_delivery_projection,
       DATEDIFF(final_delivery_projection - initial_delivery_projection) AS projection_days_revised
FROM (
    SELECT orders.order_id,
       orders.date_dispatched AS actual_dispatch_date,
       (
           SELECT projected_date 
           FROM order_delivery_projections
           WHERE order_id = orders.order_id
           ORDER BY order_delivery_projection_id ASC
           LIMIT 1
       ) AS initial_delivery_projection,
       (
           SELECT projected_date 
           FROM order_delivery_projections
           WHERE order_id = orders.order_id
           ORDER BY order_delivery_projection_id DESC
           LIMIT 1
       ) AS final_delivery_projection
   FROM orders) z

Answer 3

我花了很多时间研究这个问题，这个问题是 MySQL 的一个基本问题，这意味着如果内部查询在FROM 查询部分，即使如果内部查询位于 SELECT 部分，您也可以引用外部查询。

至于从交货日期历史记录中获取第一个和最后一个记录，我发现这个查询可以产生正确的结果和可接受的性能。

SELECT odr.order_id,
    first_change.projected_date AS initial_projected_dispatch_date,
    last_change.projected_date AS final_projected_dispatch_date
FROM order AS odr
LEFT JOIN (
    SELECT 
        order_id, 
        MIN(order_delivery_projection_id) AS first_id, 
        MAX(order_delivery_projection_id) AS last_id
    FROM order_delivery_projections
    GROUP BY order_id
) AS change_record_finder ON change_record_finder.order_id = odr.order_id
LEFT JOIN order_delivery_projections AS first_change ON first_change.order_delivery_projection_id = change_record_finder.first_id
LEFT JOIN order_delivery_projections AS last_change ON last_change.order_delivery_projection_id = change_record_finder.last_id
WHERE -- where clauses go here

Matt 的解决方案在其他 SQL 风格中也适用，但 MySQL 缺乏必要的功能来让他的正确答案发挥作用。

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