Select query question
Why don't you use the prepared ElementCriteriaModel in the category and change the query?

{% for category in craft.categories.group('whatever') %}
    {% set articles = category.journalismMainCategory.limit(7).order('postDate').find() %} 
{% endfor %}


Your category.journalismMainCategory contains an ElementCriteriaModel that will fetch all related entries you can include a limit, change the sort order and execute it, so you'll receive 7 articles with the correct order for each of your categories

Edit: link to the docs