SQL选择非空值

Question

Customer_name   Customer_street Customer_city

Lucy            Spring          Pittsfield
Lucy            Spring          Pittsfield
Brooks          Senator         Brooklyn
James           North           Rye
Vicky           Sand Hill       Woodside
Lutz            3399 Rice St    null

我正在练习一些mysql命令。我上面有一张客户表。我想列出该表中的客户城市名称，但仅列出那些非空城市名称。我试过了

select customer_city 
from customer 
WHERE customer_city IS NOT NULL;

结果仍列出空城。我错过了什么吗？谢谢你的帮助;