缺失與零

Question

我想獲得的收視率，在我的數據庫中的特定項目行填寫。並非所有項目都有1,2,3,4或5的評分;當發生這種情況時，我該如何填寫0的行？

因此，例如，當我爲1,2,3，但不是4和5的收視率，我怎麼能填補這兩個行與0的？

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 

這裏是上面的查詢上述結果，你可以看到有沒有1和2的評價，我怎麼能得到那些rating_count，percent和avg(rating)是零？

@霍根的回答是：

Answer 1

您將使用成才沿此線，以取代空值作爲0：

SELECT ISNULL(rating, 0) FROM myTable 

Answer 2

這是什麼我有工作（根據@霍根的原始查詢）：

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