$sql="SELECT id, ort_name FROM orte WHERE active='1' ORDER BY ort_name";

$sql="
            SELECT 
            orte.id, orte.ort_name, orte.active, 
            wirtschaft.ort_id 
            FROM orte 
            LEFT JOIN wirtschaft ON orte.id=wirtschaft.ort_id 
            WHERE orte.active='1' 
            ORDER BY orte.ort_name";

SELECT DISTINCT
    o.id,
    o.ort_name,
    o.active
FROM
    orte AS o
INNER JOIN
    wirtschaft AS w
ON
    o.id = w.ort_id
WHERE
    o.active = 1
ORDER BY
    o.ort_name