select convert(nvarchar(7),入职日期,120) 月份,count(工号)  人数
from tb
where datapart(year,入职日期)=2011
group by convert(nvarchar(7),入职日期,120)
having count(工号)<20

------解决方案--------------------

SQL code
select convert(varchar(7),入职日期,120) as 月份,count(1) as 人数
from 员工表
where year(入职日期)=2011
group by convert(varchar(7),入职日期,120)
having count(1)<20