如何將幾乎相同的幾個查詢合併成一個？

Question

假設我有一個order_dummy表，其中order_dummy_id, order_id, user_id, book_id, author_id存儲。你可能會抱怨我的桌子的邏輯，但我不知道需要這樣做。我想執行以下查詢。

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 

請記住，一個訂單中包括幾個相同的書籍。因此，我通過降序列出order_dummy_id並限制1，因此只顯示A BOOK的最新訂單。但我的目標是在一張桌子上以這種方式展示其他書籍。我用group by這樣的...

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

，但它只能說明order_dummy_id有上升的結果。我不知道了。期待你的善意幫助！

Answer 1

您需要normalize你的表之間的關係。 書籍ID不應排序，因爲我們每個訂單可以有多本書。您需要有一個名爲「訂購物品」的表格，並將您的書籍與訂購數量一起存儲 - 無需多行。

要回答你的問題，我可以指出你的SQL操作IN：

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

此外，根據上下文，你可以使用或操作

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

Answer 2

爲了讓多個選擇的結果一氣呵成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 
)