简单问题，请帮忙看看

Question

select firstname+'.'+lastname as name,
age = DATEDIFF(yy,birthdate,GETDATE())
from employees
where convert(int,age) > 30
order by age desc说是没有列　age

Answer 1

select firstname+'.'+lastname as name,
age = DATEDIFF(yy,birthdate,GETDATE())
from employees
where convert(int,(DATEDIFF(yy,birthdate,GETDATE()))) > 30
order by age desc

Answer 2

--這樣寫百分之百對
select firstname+'.'+lastname as name,
age = DATEDIFF(yy,birthdate,GETDATE())
from employees
where convert(int,(DATEDIFF(yy,birthdate,GETDATE()))) > 30
order by DATEDIFF(yy,birthdate,GETDATE()) desc

Answer 3

select * from 
(
select firstname+'.'+lastname as name,
age = DATEDIFF(yy,birthdate,GETDATE())
from employees
) t
where convert(int,t.age) > 30
order by t.age desc

Answer 4

select firstname+'.'+lastname as name,
age = DATEDIFF(yy,birthdate,GETDATE())
from employees
where convert(int,DATEDIFF(yy,birthdate,GETDATE())) > 30
order by age desc