在两列上使用相关子查询 [英] Use correlated subquery over two columns

问题描述

数据库小提琴

CREATE TABLE Purchasing (
    campaign VARCHAR(255),
    main_event VARCHAR(255),
    sub_event VARCHAR(255),
    quantity VARCHAR(255)
);

INSERT INTO Purchasing
(campaign, main_event, sub_event, quantity)
VALUES 
("C001", "Offered", NULL, "500"),
("C001", "Ordered", NULL, "450"),
("C001", "Storing", "Delivered", "465"),
("C001", "Storing", "Recorded", "440"),
("C001", "Storing", "Completed", "445"),

("C002", "Offered", NULL, "600"),
("C002", "Ordered", NULL, "700"),
("C002", "Storing", "Delivered", "690"),
("C002", "Storing", "Recorded", "692"),

("C003", "Offered", NULL, "300"),
("C003", "Ordered", NULL, "250"),
("C003", "Storing", "Delivered", "320"),

("C004", "Offered", NULL, "800"),
("C004", "Ordered", NULL, "870"),
("C004", "Storing", "Delivered", "740"),

("C005", "Offered", NULL, "240"),
("C005", "Ordered", NULL, "250"),
("C005", "Storing", "Delivered", "226"),

("C006", "Offered", NULL, "100"),
("C006", "Ordered", NULL, "105"),

("C007", "Offered", NULl, "900"),
("C008", "Offered", NULl, "400");

上表显示了使用main_events 和sub_events 的不同campaigns 的购买过程.
这些事件的层次结构是这样的:

The table above displays the purchasing process of different campaigns using main_events and sub_events.
The hierarchy of those events is like this:

main_event = Storing > Ordered > Offered 
sub_event = Completed > Recorded > Delivered

一旦活动在 main_event Storing 中，sub_events 适用.

Once a campaign is in the main_event Storing the sub_events apply.

现在，我想根据最高的 main_event 提取广告系列 - 或者如果他们已经有 sub_event - 基于最高的 sub_event.结果应该是这样的:

Now, I want to extract the campaigns based on their highest main_event - or in case they already have a sub_event - based on their highest sub_event. The result should look like this:

Campaign       main_event     sub_event         Quantity
C001           Storing        Completed          445
C002           Storing        Recorded           692
C003           Storing        Delivered          320
C004           Storing        Delivered          740
C005           Storing        Delivered          226
C006           Ordered        NULL               105
C007           Offered        NULL               900
C008           Offered        NULL               400

<小时>

参考这个问题我想我有组合两个相关子查询:

IF sub_event IS NULL 则使用

SELECT
campaign, 
main_event,
quantity
FROM Purchasing p
WHERE main_event = (SELECT p2.main_event
                    FROM Purchasing p2
                    WHERE p2.campaign = p.campaign
                    ORDER BY field(p2.main_event, 'Storing', 'Ordered', 'Offered')
                    LIMIT 1
                   );

其他

SELECT
campaign, 
sub_event,
quantity
FROM Purchasing p
WHERE sub_event = (SELECT p3.sub_event
                    FROM Purchasing p3
                    WHERE p3.campaign = p.campaign
                    AND p3.sub_event IS NOT NULL
                    ORDER BY field(p3.sub_event, 'Completed', 'Recorded', 'Delivered')
                    LIMIT 1
                   );

如何连接那些相关子查询以获得预期结果?

How can I connect those correlated subqueries to get the expected result?

推荐答案

如果你有主键，这会简单得多:

This would be much simpler if you had a primary key:

CREATE TABLE Purchasing (
    purchasing_id int auto_increment primary key,
    campaign VARCHAR(255),
    main_event VARCHAR(255),
    sub_event VARCHAR(255),
    quantity VARCHAR(255)
);

通过这个简单的添加:

SELECT p.*
FROM Purchasing p
WHERE p.purchasing_id = (SELECT p2.purchasing_id
                         FROM Purchasing p2
                         WHERE p2.campaign = p.campaign
                         ORDER BY field(p2.main_event, 'Storing', 'Ordered', 'Offered'),
                                  field(p2.sub_event, 'Completed', 'Recorded', 'Delivered')
                         LIMIT 1
                        );

这里是db<>小提琴.

Here is a db<>fiddle.

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