使用参数连接来自同一个表的两个字段

Question

我有两张这样的表：

标签主题

+-----------+--------------------+--------------------+----------------------+
| SubjectId | SubjectDescription | MeetingIdImportant | MeetingIdUnimportant |
+-----------+--------------------+--------------------+----------------------+
|    INT    |    NVARCHAR(100)   |      INT NULL      |       INT NULL       |
+-----------+--------------------+--------------------+----------------------+

tabMeeting

+-----------+-------------+
| MeetingId | MeetingDate |
+-----------+-------------+
|    INT    |  DATETIME   |
+-----------+-------------+

在我的应用程序中，我有一个调用存储过程的过滤器（带有SubjectIds 的下拉列表）。

存储过程用于在网格中显示数据，如下所示：

+-------------+------------------------------+--------------------------------+
| MeetingDate | Number of important subjects | Number of unimportant subjects |
+-------------+------------------------------+--------------------------------+
| 01.05.2016  |              5               |               3                |
+-------------+------------------------------+--------------------------------+

如果没有选择任何主题，则必须显示所有可能的会议日期（即使他们还没有指定任何主题）。如果选择了一个主题，那么只有那些将该主题作为重要或不重要主题的会议才会出现。

例子：

我选择了主题“测试”。这个主题在日期为 04.05.2016 的会议中很重要，在日期为 11.05.2016 的会议中不重要。现在我期待这样的结果：

+-------------+------------------------------+--------------------------------+
| MeetingDate | Number of important subjects | Number of unimportant subjects |
+-------------+------------------------------+--------------------------------+
| 04.05.2016  |              6               |               2                |
| 11.05.2016  |              2               |               4                |
+-------------+------------------------------+--------------------------------+

这是在我的存储过程中使用此代码：

SELECT meeting.MeetingId,
    meeting.MeetingDate,
    (SELECT COUNT(MeetingIdMS1) FROM dbo.tabSubject WHERE MeetingIdImportant = meeting.MeetingId) AS NumberOfImportantSubjects,
    (SELECT COUNT(MeetingIdMS8) FROM dbo.tabSubject WHERE MeetingIdUnimportant = meeting.MeetingId) AS NumberOfUnimportantSubjects
FROM dbo.tabMeeting meeting
INNER JOIN
(
    SELECT MeetingIdImportant MeetingId
    FROM dbo.tabSubject
    WHERE (@SubjectId IS NOT NULL AND SubjectId = @SubjectId)
    UNION ALL
    SELECT MeetingIdUnimportant
    FROM dbo.tabSubject
    WHERE (@SubjectId IS NOT NULL AND SubjectId = @SubjectId)
) t ON
    CASE
        WHEN @SubjectId IS NOT NULL THEN t.MeetingId
        ELSE meeting.MeetingId
    END = meeting.MeetingId

但是当我没有选择任何主题时，不会显示任何数据。我错过了什么？

Answer 1

如果@SubjectId为空且INNER JOIN没有返回数据，则表t没有数据

SELECT meeting.MeetingId,
    meeting.MeetingDate,
    (SELECT COUNT(MeetingIdMS1) FROM dbo.tabSubject WHERE MeetingIdImportant = meeting.MeetingId) AS NumberOfImportantSubjects,
    (SELECT COUNT(MeetingIdMS8) FROM dbo.tabSubject WHERE MeetingIdUnimportant = meeting.MeetingId) AS NumberOfUnimportantSubjects
FROM dbo.tabMeeting meeting
WHERE 
    @SubjectId IS NULL 
    OR EXISTS
    (
        SELECT * FROM tabSubject 
        WHERE 
        (
            MeetingIdImportant = meeting.MeetingId 
            OR MeetingIdUnimportant = meeting.MeetingId
        ) AND SubjectId = @SubjectId
    )