mysql - AVG/COUNT MySQL 查询不起作用

Question

我在 MySQL 查询方面不是很有经验，所以我可能做错了什么。 简化我的查询是这样的:

SELECT item.*, AVG(itemRating.rating) as 'rating', COUNT(itemRating.rating) as 'ratingCount' 
  FROM item, itemRating 
 WHERE item.id IN (...) 
   AND itemRating.item_fk = item.id 
 GROUP BY itemRating.item_fk

它工作正常，除非项目没有评级(itemRating 表中没有记录)。有什么办法可以在不丢失信息的情况下解决这个问题吗？

Answer 1

SELECT item.id, 
       AVG(itemRating.rating) as 'rating', 
       COUNT(itemRating.rating) as 'ratingCount' 
  FROM item
  LEFT JOIN itemRating ON itemRating.item_fk = item.id 
 WHERE item.id IN (...) 
 GROUP BY item.id