Question

假设我有一个order_dummy表，其中存储了order_dummy_id, order_id, user_id, book_id, author_id。你可能会抱怨我的桌子的逻辑，但我不知何故需要这样做。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

请注意，一个订单中包含多个相同数量的书。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。我像这样使用了group by ......

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

但它只显示order_dummy_id的升序结果。我不知道了。期待你的善意帮助！

Answer 1

要一次性获取多个选择的结果，请使用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
)

Answer 2

您需要normalize表格之间的关系。 book id不应该在订单行中，因为我们每个订单可以有多本书。您需要有一个名为“订购商品”的表格，并将您的图书与订购数量一起存储在那里 - 不需要多行。

要回答您的问题，我可以将您指向SQL运算符IN：

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

此外，根据上下文，您可以使用OR运算符

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

如何将几乎相同的几个查询合并为一个？

2 个答案: