联接表中不存在值

我正在使用此查询查找来自 users 不存在于 emails_sent 桌子

SELECT users.email,users.registration_date 
FROM users 
LEFT JOIN emails_sent
ON emails_sent.email=users.email
WHERE emails_sent.email IS NULL
ORDER BY registration_date DESC
LIMIT 0,50

相反,我想回去 users.email 没有记录的地方 emails_sent.type =“欢迎”

我尝试了下面的,但没有结果

SELECT users.email,users.registration_date 
FROM users 
LEFT JOIN emails_sent
ON emails_sent.email=users.email
WHERE emails_sent.type!='welcome'
ORDER BY registration_date DESC
LIMIT 0,50

emails_send表如下

id  int(11) unsigned    NO  PRI NULL
email   varchar(200)    YES     NULL
type    varchar(200)    YES     NULL
timestamp   int(200)    YES     NULL