用零填写缺失的行

Question

我正在尝试获取数据库中特定项目的评分。并非所有商品的评级均为1,2,3,4或5;当发生这种情况时，我如何填写0的行？

因此，例如，当我的评分为1,2,3而不是4和5时，我怎么能用0填写这两行呢？

select 
rating,
count(rating) as rating_count,
(count(rating) / (select count(item_id) from ratings where item_id = 3) * 100) as percent,
avg(rating)
from ratings 
where item_id = 3
group by rating desc
with rollup

以上是上述查询的结果，因为您可以看到没有1和2的评分，如何才能获得rating_count，percent和avg(rating)为0的评分？

@ Hogan的回答：

Answer 1

这就是我的工作（基于@ Hogan的原始查询）：

select
r_list.r as rating,
ifnull(count(rating), 0) as rating_count,
ifnull((count(rating) / (select count(item_id) from ratings where item_id = ?) * 100), 0) as percent,
ifnull(avg(rating), 0)
from (SELECT 1 AS r
UNION ALL
    SELECT 2 AS r
UNION ALL
    SELECT 3 AS r
UNION ALL
    SELECT 4 AS r
UNION ALL
    SELECT 5 AS r
) r_list
left join ratings on r_list.r = ratings.rating and item_id = ?
group by r_list.r desc
with rollup

Answer 2

您将使用沿着这一行的某些方法将空值替换为0：

SELECT ISNULL(rating, 0 ) FROM myTable