[PHP-DB] order by an average in another table (basically a "top 10" list)

[chris]

I've done a google search, but most of the results are for the part I've 
already figured out.  People that have the "top 10" sorting usually want 
money for their system (one wanted an absurd $500).

I'm working on a database application where users vote on how much they 
like a particular picture.  All of the picture information is in one table, 
and all of the votes are within another table.  The picture table's auto 
incrementing id is referenced in the ratings table.

Displaying the actual average rating of a particular photograph is easy 
enough:
"SELECT avg(rating) FROM ratings WHERE parent_id = '$id'"

But how the heck would one select the top $n entries, ordered by the 
average of the photo's ratings?  Ideally, I'd like the query to work in 
both MySQL and PostgreSQL.

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