update 
emails_1442 tm set tm.category = 'unknown.smtp.com.cn'
 where lower(trim(nvl(nvl tm.category,'unknown.smtp.com.cn'))) 
 not in (select tc.mail_category_name from EM_MAIL_CATEGORY tc); 

update emails_1442 tm
set tm.category = 'unknown.smtp.com.cn'
where not exists (select 1 from EM_MAIL_CATEGORY tc where tc.mail_category_name = tm.category);

------解决方案--------------------
我觉得应该是这样的：
SQL code

UPDATE emails_1442 tm SET tm.category = 'unknown.smtp.com.cn'
WHERE to_lower(tm.category) NOT IN 
(SELECT to_lower(tc.mail_category_name) FROM EM_MAIL_CATEGORY tc);

------解决方案--------------------
to_lower(tc.mail_category_name)-> lower(tc.mail_category_name)

------解决方案--------------------
类似于此，当然如果是字符串的话，需要注意大小写
SQL code

update  emp4  e set e.deptno = 0 where not exists (select 1 from dept d where e.deptno = d.deptno)