php – MySQL中的条件JOIN语句

我有以下工作MySQL查询：

SELECT 
    a.id id,a.price price,a.stock stock,a.max_per_user max_per_user,a.purchased purchased,b.quantity owned 
FROM 
    shop_items a 
        JOIN shop_inventory b 
            ON b.iid=a.id 
            AND b.cid=a.cid 
WHERE 
    a.cid=1 
    AND a.szbid=0 
    AND a.id IN(3,4)

JOIN加入表shop_inventory b以返回b.quantity所拥有的.但是,如果shop_inventory b表中没有记录,其中b.iid = a.id我希望它返回b.quantity = 0.我该怎么做？
最佳答案
请改用LEFT JOIN.和COALESCE因为一些记录为null(我猜).尝试,

SELECT a.id id,COALESCE(b.quantity,0) owned 
FROM shop_items a 
          LEFT JOIN shop_inventory b 
                ON b.iid=a.id AND b.cid=a.cid 
WHERE a.cid=1 AND 
      a.szbid=0 AND 
      a.id IN(3,4)