为什么在 Oracle SQL 中为子查询添加别名时不能使用 AS？

Question

我正在处理此查询并遇到“命令未正确结束”错误。 我发现这是由于我如何为子查询别名。

select ctable.code, company.founder, ctable.lman, ctable.sman, ctable.man, ctable.emp
from (select 
c.company_code as code, 
count(distinct l.lead_manager_code) as lman, 
count(distinct s.senior_manager_code) as sman,
count(distinct m.manager_code) as man,
count(distinct e.employee_code) as emp
from company c join lead_manager l on c.company_code = l.company_code
join senior_manager s on c.company_code = s.company_code
join manager m on c.company_code = m.company_code
join employee e on c.company_code = e.company_code 
group by c.company_code) AS ctable 
join company on company.company_code = ctable.code
order by ctable.code;

我发现如果我删除子查询末尾的“AS”，错误就会消失，如下所示：

select ctable.code, company.founder, ctable.lman, ctable.sman, ctable.man, ctable.emp
from (select 
c.company_code as code, 
count(distinct l.lead_manager_code) as lman, 
count(distinct s.senior_manager_code) as sman,
count(distinct m.manager_code) as man,
count(distinct e.employee_code) as emp
from company c join lead_manager l on c.company_code = l.company_code
join senior_manager s on c.company_code = s.company_code
join manager m on c.company_code = m.company_code
join employee e on c.company_code = e.company_code 
group by c.company_code) ctable 
join company on company.company_code = ctable.code
order by ctable.code;

据我所知，AS 运算符只是一种以更详细的方式设置别名的方法，那么为什么在尝试使用 AS 与不使用 AS 为子查询设置别名时会失败呢？ 当我在子查询中使用 AS 为列名添加别名时，它就可以工作。

Answer 1

语法不允许。

来自

SELECT

文档：

table_reference::= 

AS

和

query_table_expression

之间没有

t_alias

。