如何将几乎相同的几个查询合并成一个？

Question

假设我有一个order_dummy表，其中order_dummy_id, order_id, user_id, book_id, author_id存储。你可能会抱怨我的桌子的逻辑，但我不知道需要这样做。我想执行以下查询。

SELECT * 
FROM order_dummy 
WHERE order_id = 1 
AND user_id = 1 
AND book_id = 1 
ORDER BY `order_dummy_id` DESC 
LIMIT 1 

SELECT * 
FROM order_dummy 
WHERE order_id = 1 
AND user_id = 1 
AND book_id = 2 
ORDER BY `order_dummy_id` DESC 
LIMIT 1 

SELECT * 
FROM order_dummy 
WHERE order_id = 1 
AND user_id = 1 
AND book_id = 3 
ORDER BY `order_dummy_id` DESC 
LIMIT 1 

请记住，一个订单中包括几个相同的书籍。因此，我通过降序列出order_dummy_id并限制1，因此只显示A BOOK的最新订单。但我的目标是在一张桌子上以这种方式展示其他书籍。我用group by这样的...

SELECT * 
FROM order_dummy 
WHERE order_id = 1 
AND user_id = 1 
GROUP BY book_id 

，但它只能说明order_dummy_id有上升的结果。我不知道了。期待你的善意帮助！

Answer 1

您需要normalize你的表之间的关系。 书籍ID不应排序，因为我们每个订单可以有多本书。您需要有一个名为“订购物品”的表格，并将您的书籍与订购数量一起存储 - 无需多行。

要回答你的问题，我可以指出你的SQL操作IN：

select * from table where id in (1,2,3) 

此外，根据上下文，你可以使用或操作

select * from table where id = 1 or id = 2 or id =3 

Answer 2

为了让多个选择的结果一气呵成UNION ALL：

(
    SELECT * 
    FROM order_dummy 
    WHERE order_id = 1 
    AND user_id = 1 
    AND book_id = 1 
    ORDER BY `order_dummy_id` DESC 
    LIMIT 1 
) 
UNION ALL 
(
    SELECT * 
    FROM order_dummy 
    WHERE order_id = 1 
    AND user_id = 1 
    AND book_id = 2 
    ORDER BY `order_dummy_id` DESC 
    LIMIT 1 
) 
UNION ALL 
(
    SELECT * 
    FROM order_dummy 
    WHERE order_id = 1 
    AND user_id = 1 
    AND book_id = 3 
    ORDER BY `order_dummy_id` DESC 
    LIMIT 1 
)