Craft CMS: Filter entries by category of QueryString is found

craft3-cms-filter-entries-by-category-based-on-querystring

{# get url parameter 'filter' #}
{% set filterQuery = craft.request.getParam('filter') %}

{# if we have a query string, select these categories and find related entries #}
{% if filterQuery|length %}
    {% set relatedCats = craft.categories.group('verguenstigungen').slug(filterQuery) %}

    {# setup search parameters #}
    {% set searchParams = {
      relatedTo:  relatedCats,
      section: 'verguenstigungen'
    } %}

    {# Find all entries related to categories #}
    {% set entries = craft.entries(searchParams).order('score') %}

{# if we have no query string, find categories in category group #}
{% else %}
    {% set relatedCats = craft.categories().group('verguenstigungen') %}

    {# get all related entries #}
    {% set entries = craft.entries({
      section: 'verguenstigungen',
      orderBy: 'title asc, elements.id asc',
      limit: 10
    }) %}

{% endif %}