Join two fields from same table with parameter - sql-server

Question

I have two table like this:
tabSubject
+-----------+--------------------+--------------------+----------------------+
| SubjectId | SubjectDescription | MeetingIdImportant | MeetingIdUnimportant |
+-----------+--------------------+--------------------+----------------------+
| INT | NVARCHAR(100) | INT NULL | INT NULL |
+-----------+--------------------+--------------------+----------------------+
tabMeeting
+-----------+-------------+
| MeetingId | MeetingDate |
+-----------+-------------+
| INT | DATETIME |
+-----------+-------------+
In my application I have a filter (dropdownlist with SubjectIds) that calls a stored procedure.
The stored procedure is used to show data in a grid like this:
+-------------+------------------------------+--------------------------------+
| MeetingDate | Number of important subjects | Number of unimportant subjects |
+-------------+------------------------------+--------------------------------+
| 01.05.2016 | 5 | 3 |
+-------------+------------------------------+--------------------------------+
If no subject is chosen, all possible meeting dates have to be shown (even if they don't have any assigned subject yet). If a subject is chosen, only those meetings should appear that have this subject as either important or unimportant subject.
Example:
I chose subject "Testing". This subject is important in the meeting with the date 04.05.2016 and unimportant in the meeting with the date 11.05.2016. Now I expect a result like this:
+-------------+------------------------------+--------------------------------+
| MeetingDate | Number of important subjects | Number of unimportant subjects |
+-------------+------------------------------+--------------------------------+
| 04.05.2016 | 6 | 2 |
| 11.05.2016 | 2 | 4 |
+-------------+------------------------------+--------------------------------+
This is working with this code in my stored procedure:
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
But when I don't chose any subject, no data is shown. What am I missing out?

Answer 1

The table t has no data if #SubjectId is null and INNER JOIN returns no data
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
)