計數返回NULL時，它應該只是零

Question

我有一個表跟蹤用戶的可信度。

+-----------------+-----------------+-------------+ 
| user_id_1  | user_id_2  | is_trusted | 
+-----------------+-----------------+-------------+ 
| 1    | 2    | 0   | 
... 

其中1是可信的，0不是。如果沒有負面反饋，我會得到NULL的值。有沒有辦法獲得積極的 - 0？

select tr2.user_id_2 user, 
    ((select count(*) plus 
     from trust_ratings tr 
     where is_trusted = 1 and 
     tr2.user_id_2 = tr.user_id_2 
     group by tr.user_id_2) 
    - 
    (select count(*) minus 
     from trust_ratings tr 
     where is_trusted = 0 and 
     tr2.user_id_2 = tr.user_id_2 
     group by tr.user_id_2)) as score 
    from trust_ratings tr2 
    group by user; 

Answer 1

你可以使用case when：

select user_id_2 user, 
     sum(case is_trusted when 1 then 1 else -1 end) as score 
    from trust_ratings 
group by user; 

或者：

select user_id_2 user, 
     sum(is_trusted * 2 - 1) as score 
    from trust_ratings 
group by user; 

Answer 2

使用COALESCE()：

select tr2.user_id_2 user, 
    (
     coalesce(
     (select count(*) plus 
     from trust_ratings tr 
     where is_trusted = 1 and 
     tr2.user_id_2 = tr.user_id_2 
     group by tr.user_id_2), 0) 
    - 
     coalesce(
     (select count(*) minus 
     from trust_ratings tr 
     where is_trusted = 0 and 
     tr2.user_id_2 = tr.user_id_2 
     group by tr.user_id_2), 0) 
    ) as score 
    from trust_ratings tr2 
    group by user;