sql - MySQL GROUP BY 数量

Question

MySQL表是这样的(VoteID是PK):

VoteID  VoteValue CommentID
1         -1         1
2         -1         1
3          1         1
4         -1         1
5          1         2
6          1         2
7         -1         2

我需要一个结果:

CommentID    Plus    Minus
    1          1      3 
    2          2      1 

按 CommentID 分组的“优点”总和、“缺点”总和 是否可以通过一个 SQL 表达式获得所需的结果？

Answer 1

SELECT 
    CommentID,
    SUM(CASE WHEN VoteValue > 0 THEN 1 ELSE 0 END) AS PLUS,
    SUM(CASE WHEN VoteValue < 0 THEN 1 ELSE 0 END) AS MINUS

FROM 
    mytable

GROUP BY 
    CommentID