SELECT c_nickname FROM customer 
WHERE c_nickname = 'willow' 
UNION 
SELECT v_nickname FROM vendor 
WHERE v_nickname = 'willow' 

SELECT IF(COUNT(c_nickname) > 0 , 'YES', 'NO' ) AS Ergebnis 
FROM customer 
WHERE c_nickname = 'willow' 
UNION 
SELECT IF(COUNT(v_nickname) > 0 , 'YES', 'NO' ) AS Ergebnis 
FROM vendor 
WHERE v_nickname = 'willow'