如何禁止在查询中两次使用相同的表别名？

Question

我在跑步Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production Version 19.8.0.0.0。

考虑以下示例：

create table tab_a as
          select 1 as id from dual
union all select 2 as id from dual
union all select 3 as id from dual
;

create table tab_b as
          select 1 as id, 'b1' as val, 'bbb1' as val_b from dual
union all select 2 as id, 'b2' as val, 'bbb2' as val_b from dual
;

create table tab_c as
          select 1 as id, 'c1' as val, 'ccc1' as val_c from dual
union all select 3 as id, 'c3' as val, 'ccc1' as val_c from dual
;

select
  a.id
 ,b.val
 ,b.val_b
 ,b.val_c
from
  tab_a a
  left join tab_b b on b.id = a.id
  left join tab_c b on b.id = a.id
order by
  a.id
;

正如您所看到的，我在另一个表上重复使用了“b”别名（重复）。这是结果：

        ID VAL        VAL_2      VAL_3
---------- ---------- ---------- ----------
         1 b1         bbb1       ccc1
         2 b2         bbb2       (null)
         3 (null)     (null)     ccc1

所以：

• 两个表中都存在的列“val”取自首先声明的 tab_b
• 列“val_b”取自 tab_b
• 列“val_c”取自 tab_c

看来 Oracle 为两个表分配了优先级别名，并且不会将其报告为语法错误。但这是我想避免的。我很惊讶在我的查询中发现了这个错误。这种行为是已知的吗？可以采取什么措施来强制使用唯一的别名吗？

Answer 1

由于我的声誉，我无法发表评论，但最近遇到了这个问题，并想提及已在 23c 上修复。

SQL> select
  2    a.id
  3   ,b.val
  4   ,b.val_b
  5   ,b.val_c
  6  from
  7    tab_a a
  8    left join tab_b b on b.id = a.id
  9    left join tab_c b on b.id = a.id
 10  order by
 11    a.id
 12  ;
 ,b.val
  *
ERROR at line 3:
ORA-00918: column ambiguously defined