求一条sql语句

Question

数据库表名 employee 结构如下；
id   employeeid   flag
1       x          0
2       x          1
3       y          0
4       z          1
5       m          0
6       n          0
查询出表中只有flag=0的employeeid，结果应该为：
id   employeeid   flag
3       y          0
5       m          0
6       n          0
请问大家 sql语句怎么写啊？效率越高越好

Answer 1

select id, exployeeID, flag
from employee 
where flag='0'

Answer 2

select id, exployeeID, flag
from employee 
where id not in(select id from employee where flag<>0)

Answer 3

select
  em.id,
  em.exployeeID,
  em.flag
from
  employee em
where
  not exists(
    select
      null
    from
      employee ep
    where
      ep.exployeeID = em.exployeeID
      and ep.flag = 0)

Answer 4

select id, employeeid, flag
from employee 
where flag='0' and employeeid not in (select employeeid from employee where flag<>'0')

Answer 5

select * from employee where employeeid in(select employeeid from employee where flag='0' group by employeeid having count(*)=1)

Answer 6

select id, employeeID, flag
from employee emp
where not exists (select 1 from employee where employeeid=emp.employeeid and flag<>0);