你可以通过将你的条件作为子查询传递，然后将left join应用到你的表中：

select s.user, t.code
from (
  select 'user1' as user union all 
  select 'user2' union all 
  select 'user3' union all 
  select 'user4' 
) as s
left join mytable t on s.user = t.user;

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结果如下：
| 用户|代码|
| --|--|
| user1|代码1|
| user2| null|
| user3|代码2|
| 用户4|代码4|
Demo here

可以直接在where子句中过滤

select user,code
from (
  select 'user1' as user, 'abc' as code union all 
  select 'user2','def' union all 
  select 'user3', 'ghi' union all 
  select 'user4' ,NULL
) as s
where code is null;

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