Select newest TOPIC for each SUBJECT in SQL Server 2008 - sql

Question

I have a project about programming a forum.
I have 2 tables in the database:
SUBJECT (idSUB, titleSUB);
TOPIC (idTOP, titleTOP, contentTOP, idSUB, idUser [user create topic], Time);
All I want is:
+ select COUNT(*) from TOPIC as numTOPIC group by idSUB--> as Table A
+select TOP 1 titleTOP order by Time desc-> as newestTOP group by idSUB---> as Table B
+ Then JOIN 3 table A,B,SUBJECT--> C(idSUB,titleSUB,numTOPIC,newestTOP, idUser (who created the newest topic))
I've found the way to LEFT JOIN A,SUBJECT-> C(idSUB,titleSUB,numTOPIC) but I really don't know the right syntax to JOIN 3 Tables above.
SELECT
a.idSUB, a.titleSUB,
COUNT(b.idSUB) numTOPIC
FROM
SUBJECT a
LEFT JOIN
TOPIC b ON a.idSUB = b.idSUB
GROUP BY
a.idSUB, a.titleSUB
I just want do this in only one query. Help!
UPDATE:
By the code of #John Bingham below, the output table cannot display the SUBJECT which doesn't have any TOPIC. I want all the TOPIC can be display.
SELECT
s.idSUB, s.titleSUB, a.numTOPIC,
isnull(b.newestTOP, '') as [Newest Topic],
isnull(b.idUser, '')
FROM
Subject s
INNER JOIN
(SELECT
IDSub, Count(*) as NumTopic
FROM
Topic
GROUP BY IDSub) a ON s.IDSub = a.IDSub
LEFT JOIN
(SELECT
t.IDSub, t.titleTop as newestTop, t.idUser as [idUser]
FROM
Topic t
INNER JOIN
(SELECT IDSub, Max([Time]) as tm
FROM Topic
GROUP BY IDSub) x ON t.IDSub = x.IDSub
WHERE t.[Time] = x.tm) b ON s.IDSub = b.IDSub
It's the right query but I want more exactly, help!

Answer 1

My inclination is to convert each of the requirements (a), (b) & (c) into subqueries which deliver the virtual tables to your query, rather than attempting to combine the base tables to deliver the requirements in a singe hit. So -
SELECT s.idSUB, s.titleSUB, a.numTOPIC, isnull(b.newestTOP, '') as [Newest Topic],
isnull(b.idUser, '')
FROM Subject s
INNER JOIN (SELECT IDSub, Count(*) as NumTopic FROM Topic GROUP BY IDSub) a
ON s.IDSub = a.IDSub
LEFT JOIN (
SELECT t.IDSub, t.titleTop as newestTop, t.idUser as [idUser]
FROM Topic t
INNER JOIN (
SELECT IDSub, Max([Time]) as tm FROM Topic GROUP BY IDSub
) x ON t.IDSub = x.IDSub
WHERE t.[Time] = x.tm
) b ON s.IDSub = b.IDSub

Answer 2

You want to display idSub,titleSub,numTopic,newestTOP,idUser where numTopic is number of topics in any subject i.e idSub and newestTOP is latest topic for the same subject.
I am using MySQL and tested the following query and it works fine.
SELECT S.idSub, S.titleSUB, TOPIC_COUNT_TABLE.NUMTOPIC, NEWESTTOPIC_TABLE.NEWESTTOPIC,
NEWESTTOPIC_TABLE.IDUSER
FROM SUBJECT S,
(SELECT IDSUB, COUNT(*) AS NUMTOPIC FROM TOPIC
GROUP BY TOPIC.IDSUB) AS TOPIC_COUNT_TABLE,
(SELECT T.IDSUB,T.titleTOP AS NEWESTTOPIC, T.IDUSER
FROM TOPIC T,(SELECT IDTOP,IDSUB,MAX(TIME) AS MAXTIME FROM TOPIC
GROUP BY IDSUB ) AS MAXTIME_TABLE
WHERE T.TIME = MAXTIME_TABLE.MAXTIME) AS NEWESTTOPIC_TABLE
WHERE S.IDSUB = NEWESTTOPIC_TABLE.IDSUB AND S.IDSUB = TOPIC_COUNT_TABLE.IDSUB;