php - MySQL 中的条件 JOIN 语句

Question

我有以下有效的 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 连接表shop_inventory b 以返回b.quantity owned。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？

Answer 1

改用LEFT JOIN。并且 COALESCE 因为一些记录为空(我猜)。试试看，

SELECT a.id id,a.price price,a.stock stock,
       a.max_per_user max_per_user,a.purchased purchased, 
       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)