MySQL的：INNER JOIN崩潰2 LEFT JOIN的，如果他們中的一個將返回NULL

Question

查詢：

SELECT 
    i.id, i.title, i.description, 
    cities.name as city, 
    GROUP_CONCAT(DISTINCT station.name) as station, 
    GROUP_CONCAT(DISTINCT p.url) as photos 
FROM 
    items i 
INNER JOIN 
    cities ON cities.id = i.city_id 
LEFT JOIN 
    item_photos p ON p.item_id = i.id 
LEFT JOIN 
    item_stations s ON s.item_id = i.id 
INNER JOIN 
    stations ON stations.id = s.station_id 
WHERE i.id = ? 
LIMIT 1 

如果表item_stations行不存在，無論是LEFT JOIN的作品：返回站的照片和NULL。但在這種情況下INNER JOIN查詢將返回NULL爲照片和電臺。我應該如何重寫查詢以說INNER JOIN不要連接表，如果item_stations中沒有需要的行s.item_id = i.id？

Answer 1

如果你真的需要做的內蒙古item_stations和stations表之間的連接，那麼你可能要考慮使用INNER JOIN子查詢：

SELECT 
    i.id, i.title, i.description, 
    cities.name as city, 
    GROUP_CONCAT(DISTINCT s.name) as station, 
    GROUP_CONCAT(DISTINCT p.url) as photos 
FROM items i 
INNER JOIN cities 
    ON cities.id = i.city_id 
LEFT JOIN item_photos p 
    ON p.item_id = i.id 
LEFT JOIN 
(
    select item_id, name 
    from item_stations s 
    INNER JOIN stations 
    ON stations.id = s.station_id 
) 
    ON s.item_id = i.id 
WHERE i.id = ? 
LIMIT 1 

否則我會使用LEFT JOIN建議兩者之間：

SELECT 
    i.id, i.title, i.description, 
    cities.name as city, 
    GROUP_CONCAT(DISTINCT stations.name) as station, 
    GROUP_CONCAT(DISTINCT p.url) as photos 
FROM items i 
INNER JOIN cities 
    ON cities.id = i.city_id 
LEFT JOIN item_photos p 
    ON p.item_id = i.id 
LEFT JOIN item_stations s 
    ON s.item_id = i.id 
LEFT JOIN stations 
    ON stations.id = s.station_id 
WHERE i.id = ? 
LIMIT 1