T-SQL Select and count from different table?

Question

I have a table (Threads) containing a field (id). I would like to select every row from Threads, as well as the number of rows in the

Answer 1

SELECT t.id, COUNT(p.thread)
FROM Threads AS t
    LEFT OUTER JOIN Posts AS p
        ON t.id = p.thread
GROUP BY t.id

Answer 2

Sure - something like this?

SELECT 
    t.ThreadID,
    (SELECT COUNT(*) FROM dbo.Posts p WHERE p.ThreadID = t.ThreadID)
FROM
    dbo.Threads t