我什么时候可以/应该在 SQL 中使用别名？

Question

我有以下代码（可以正常工作）：

with t as 
        (select *,

         dense_rank() over (partition by departmentId order by salary desc) as r 
       from employee
  )


select d.name as Department, t.name as Employee, t.Salary

from (t left join department d on t.departmentid=d.id) 

where 
    t.r <=3
    

我不明白的是，如果我尝试为连接表提供别名，为什么会出现错误：

with t as 
        (select *,

         dense_rank() over (partition by departmentId order by salary desc) as r 
       from employee
  )


select d.name as Department, t.name as Employee, t.Salary

from (t left join department d on t.departmentid=d.id) as x

where 
    x.r <=3
    

我知道别名不是必需的，但我不明白为什么它会出错。

我希望代码能够与别名一起正常工作

Answer 1

(t left join department d on t.departmentid=d.id)

不是一个表格引用，它是两个表格引用。

t

和

left join department d on t.departmentid=d.id

。给它们统一起一个别名是没有意义的，

t.r

和

d.name

是明确的。

括号是允许的，但不是必需的，并且暗示不存在的分组。摆脱它们更简单。

with t as (
  select 
    *,
    dense_rank() over (
      partition by departmentId 
      order by salary desc
    ) as r 
  from employee
)
select 
  d.name as Department, 
  t.name as Employee, 
  t.Salary
from t
left join department d on t.departmentId=d.id
where t.r <=3

参见 15.2.13.2 JOIN 子句