T-SQL Select and count from different table?

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 table Posts where the field Posts.thread i开发者_开发问答s the same as Threads.id.

How can this be done in SQL?

(Something like this pseudo-SQL: SELECT *, COUNT(* FROM Posts WHERE Posts.id=Threads.id) FROM Threads)

Sure - something like this?

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

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