I need to update a column that tracks the number of "child" posts in a forum. Both Parent and child posts are stored in the same table. The schema is:

```
ForumPosts {
   PostID: bigint,
   ParentFK: bigint, -- if child, this will point to the parent
   AnswerCount: int
   ...
}

```

If the post is a child, **parentFK** points to a different record in the **ForumPosts** table.

I want to do something like this:

```
UPDATE ForumPosts 
SET AnswerCount = (
        SELECT COUNT(PostID) 
        FROM ForumPosts 
        WHERE ParentFK = ???
)

```

... but it doesn't work because the SELECT needs to reference the record being updated, which I'm not sure how to do.