有没有办法根据 MySQL 中的 CASE 动态定义运算符？

Question

我有一个查询，它自行连接以获取每个唯一列表的最新或最旧订单记录。

SELECT
    lists.ordered,
    cnt1.*
FROM 
    cnt_lists as cnt1
    LEFT JOIN 
        lists
        on 
            cnt1.list_id = lists.id
    LEFT JOIN 
        cnt_lists as cnt2
        on 
            (cnt1.list_id = cnt2.list_id AND cnt1.id < cnt2.id)

WHERE
    cnt2.id is null and cnt1.list_id in ('3176', '3295', '3296') and cnt1.listable_type = 'Movie';

这个查询很好用，但是lists.ordered 可以是 0 或 1。当lists.ordered = 0 我希望on 语句中的运算符为cnt1.id < cnt2.id 但当lists.ordered = 1 我想要它被反转cnt1.id > cnt2.id。

有没有办法根据CASE 语句动态定义运算符？下面的方法不起作用，但我在玩想法。

SELECT
    lists.ordered,
    CASE
        WHEN lists.ordered = 1 THEN '>'
        ELSE '<'
    END AS operator,
    cnt1.*
FROM 
    cnt_lists as cnt1
    LEFT JOIN 
        lists
        on 
            cnt1.list_id = lists.id
    LEFT JOIN 
        cnt_lists as cnt2
        on 
            (cnt1.list_id = cnt2.list_id AND cnt1.id operator cnt2.id)

WHERE
    cnt2.id is null and cnt1.list_id in ('3176', '3295', '3296') and cnt1.listable_type = 'App\\Models\\Movie';

如何为每个列表同时提取最高顺序和最低顺序，我可以确定在 PHP 端使用这两个记录中的哪一个？

我只是在寻找想法，因为我试图避免必须单独查询每个列表的 N + 1 查询问题。

Answer 1

依赖动态查询很麻烦。您最好将该运算符拉入查询的 WHERE 部分并将其与 OR 配对：

SELECT
    lists.ordered,
    cnt1.*
FROM 
    cnt_lists as cnt1
    LEFT JOIN 
        lists
        on 
            cnt1.list_id = lists.id
    LEFT JOIN 
        cnt_lists as cnt2
        on 
            (cnt1.list_id = cnt2.list_id)

WHERE
    cnt2.id is null and cnt1.list_id in ('3176', '3295', '3296') 
    and cnt1.listable_type = 'Movie'
    AND (
       cnt2.id IS NULL /* Including this here because you have a LEFT JOIN */
       (lists.ordered = 1 AND cnt1.id < cnt2.id) OR 
       (lists.ordered = 0 AND cnt1.id > cnt2.id)
    );

您的另一个选择是直接将该逻辑放入连接逻辑中。这可能更容易阅读。

SELECT
    lists.ordered,
    cnt1.*
FROM 
    cnt_lists as cnt1
    LEFT JOIN 
        lists
        on 
            cnt1.list_id = lists.id
    LEFT JOIN 
        cnt_lists as cnt2
        on 
            (cnt1.list_id = cnt2.list_id AND (
       (lists.ordered = 1 AND cnt1.id < cnt2.id) OR 
       (lists.ordered = 0 AND cnt1.id > cnt2.id)
    ))

WHERE
    cnt2.id is null and cnt1.list_id in ('3176', '3295', '3296') 
    and cnt1.listable_type = 'Movie'
    ;

您可以选择在最终输出中包含 <，但这不会特别有用。

如果你*REALLY*想要创建一个动态查询，很难一步完成，but it is definitely doable。