Question

昨天我遇到了一个客户端的查询，它类似于：

select count(*) as countall,
          person,
          location 
  from (select custid,
               min("Date") as theDate,
               person,
               data.location  
        from data join 
             customer on customer.customerid = custid 
        where status = 1 
          and custid <> -1 
       group by custid,
                person,
                data.location)  --[NO ALIAS]
   group by person,location

我在Oracle中没有很多时间，但据我所知，在MS SQL中这不会飞。每当我使用派生表时，它都会抛出一个错误，所以遇到这样的场景会引起我的兴趣。我无法在interwebs上找到任何解释它的东西，希望有人可以解释派生表的别名是可选的以及何时不是。

Answer 1

当您引用未唯一定义的列时，您只需要别名。这意味着该列存在于多个表/派生表中。引用可以在select语句或连接中。如果所有列都是唯一的，那么您不需要别名。

为了清晰起见，我更喜欢使用别名，因为它有助于PL / SQL中的Intellisense。

--ALIAS needed, because the 'a' column referenced is not unique
--this will throw an error
select a, a, b, c
  from (select 'A1' as a, 'B1' as b, 'C1' as c from dual),
       (select 'A2' as a from dual);
--this will not throw an error
select t1.a, t2.a, b,c
  from (select 'A1' as a, 'B1' as b, 'C1' as c from dual) t1,
       (select 'A2' as a from dual) t2;
;

--ALIAS not needed for join, because all referenced columns are unique
select a, b, c, d, e, f
  from (select 'A' as a, 'B' as b, 'C' as c from dual)
  join (select 'D' as d, 'E' as e, 'F' as f from dual)
    on a = d;

--ALIAS needed for join, because the 'x' column referenced is not unique
--this will throw an error
select a
  from (select 'A' as a, 'B' as b, 'C' as c, 'X' as x from dual)
  join (select 'D' as d, 'E' as e, 'F' as f, 'X' as x from dual)
    on x = x;
--this will not throw an error
select a
  from (select 'A' as a, 'B' as b, 'C' as c, 'X' as x from dual) t1
  join (select 'D' as d, 'E' as e, 'F' as f, 'X' as x from dual) t2
    on t1.x = t2.x;

Answer 2

在这种情况下，您正在从子查询中选择所有列，因此子查询没有别名。

如果要将此子查询与另一个表或可能是另一个子查询连接，则需要使用别名，以便可以使用定义的别名引用连接列。

Oracle sql派生表 - 可选别名

2 个答案: