SQL-无法显示包含0的计数值

Question

有 2 Table 个讨论和评论，讨论 table 有 D1 和 D2 评论 table 有 (C1,D2),(C2, D2)

SELECT        Discussion.DiscussionID, COUNT(Comment.CommentID) AS Expr1
FROM            Comment INNER JOIN
                     Discussion ON Comment.DiscussionID = Discussion.DiscussionID INNER JOIN
                     Category ON Discussion.CategoryID = Category.CategoryID
GROUP BY Discussion.DiscussionID

我想要这样的输出...

Dicussion ID||   Comment COunt
D1                  0
D2                  2

但是结果是这样的

Dicussion ID||   Comment COunt
D2                  2

Answer 1

将 Inner Join 转换为 Right Join 因此将从 Discussion table.

中获取所有行

SELECT Discussion.DiscussionID,
       Count(Comment.CommentID) AS Expr1
FROM   Comment
       RIGHT JOIN Discussion
               ON Comment.DiscussionID = Discussion.DiscussionID
       INNER JOIN Category
               ON Discussion.CategoryID = Category.CategoryID
GROUP  BY Discussion.DiscussionID

Answer 2

或使用左连接

SELECT Discussion.DiscussionID,
       Count(Comment.CommentID) AS Expr1
FROM   Discussion
       INNER JOIN Category
               ON Discussion.CategoryID = Category.CategoryID
       LEFT JOIN Comment
               ON Comment.DiscussionID = Discussion.DiscussionID
GROUP  BY Discussion.DiscussionID

SQL-无法显示包含0的计数值

SQL-Unable to display count value that contain 0

mysql

sql

sql-server

inner-join

query-builder