MySQL JOIN返回意外值

Question

我正在嘗試做一個簡單的mysql請求，我遇到了問題。

我有一個像下面定義的2個表：

currencies 
______________________________________________________________________________________ 
currency_id | currency_name | currency_symbol | currency_active | currency_auto_update 

exchange_rates 
____________________________________________________________________________________________________ 
exchange_rate_id | currency_id | exchange_rate_date | exchange_rate_value | exchange_rate_added_date 

我想要做的是選擇內部exchange_rates最後一排的積極貨幣。

我做了這樣的：

SELECT c.currency_id, c.currency_name, c.currency_symbol, c.currency_active, er.exchange_rate_id, er.exchange_rate_date, er.exchange_rate_value 
FROM currencies c 
LEFT JOIN (
    SELECT er1.exchange_rate_id, er1.currency_id, er1.exchange_rate_date, er1.exchange_rate_value 
    FROM exchange_rates er1 
    ORDER BY er1.exchange_rate_date DESC 
    LIMIT 1 
    ) AS er 
ON er.currency_id=c.currency_id 
WHERE c.currency_active='1' 

這是我返回值NULL從exchange_rates表，即使有匹配的行

我試圖刪除LIMIT 1，但如果我這樣做像這樣將所有行返回給活動貨幣，這不是我想要的解決方案

該查詢應該如何看起來像？

謝謝！

Answer 1

試試這個：

SELECT c.currency_id, c.currency_name, c.currency_symbol, c.currency_active, er.exchange_rate_id, er.exchange_rate_date, er.exchange_rate_value 
FROM currencies c 
LEFT JOIN (SELECT * FROM 
    (SELECT er1.exchange_rate_id, er1.currency_id, er1.exchange_rate_date, er1.exchange_rate_value 
    FROM exchange_rates er1 
    ORDER BY er1.exchange_rate_date DESC) AS A GROUP BY currency_id 
    ) AS er 
ON er.currency_id=c.currency_id 
WHERE c.currency_active='1' 

Answer 2

查詢的想法看起來是正確的。您的左連接查詢應返回具有最新匯率的行。你可以在這裏使用內部連接，因爲必須有一個貨幣指的這個...

SELECT c.currency_id, c.currency_name, c.currency_symbol, c.currency_active, er.exchange_rate_id, er.exchange_rate_date, er.exchange_rate_value 
FROM currencies c 
INNER JOIN exchange_rates_er1 e ON e.currency_id = c.currency_id 
WHERE c.currency_active='1' 
ORDER BY e.exchange_rate_date DESC 
LIMIT 1