Question

Query:

SELECT * FROM #__content WHERE featured=1 and catid=$category_id or catid IN 
( SELECT id FROM #__categories WHERE parent_id=$category_id )

Featured=1 works only here catid=$category_id. The place where starts this catid IN
( SELECT id FROM #__categories WHERE parent_id=$category_id ) outputs all rows and featured=1 is ignored at this place. How to add featured=1 condition to catid IN?

So basically it looks like this by now:

$query = "SELECT * FROM #__content WHERE featured=1 and catid=$category_id";
$query2 = "SELECT * FROM #__content WHERE catid IN 
    ( SELECT id FROM #__categories WHERE parent_id=$category_id )";

Answer 1

You need to add parentheses to force the order of evaluation that you need:

SELECT * FROM #__content WHERE catid=$category_id OR (featured=1 AND catid IN 
( SELECT id FROM #__categories WHERE parent_id=$category_id ))

Since AND has higher precedence than OR, the expression without the parentheses was true when (featured=1 and catid=$category_id) was true, or catid was in the IN list specified by the subquery.