Get a specific number of rows from parent table?
Here's one way to do what you describe:

SELECT &lt;columns&gt;
FROM (
    SELECT &lt;columns&gt; 
    FROM foo
    WHERE &lt;condition&gt;     
    ORDER BY &lt;expression-list-1&gt; 
    LIMIT 15
) AS T 
LEFT JOIN bar 
    ON T.a = bar.a 
ORDER BY &lt;expression-list-2&gt; ;


You basically get 15 rows from foo according to some criteria and join the result with bar.

The first, internal ORDER BY list affects which 15 rows will be selected. The second, final list affects only the presentation ordering of the results.

The two ORDER BY lists do not need to be identical, although in practice they'll often be the same or the second an extension of the first, to resolve ties (how the children of the same parent are ordered).