如何从子选择中选择未混淆的数字文字 [英] How to select an unaliased numeric literal from a sub-select
问题描述
我想知道是否存在一种通用的SQL语法，允许从子选择中选择未混淆的数字文字：
I'm wondering if there is a general SQL syntax allowing for selecting unaliased numeric literals from sub-selects:
-- Seems to work in MySQL / Oracle
select table_alias."1"
from (
select 1 from dual
) table_alias
我知道我可以为子选择中的字段加上别名：
I know I could alias the fields in the subselect:
-- Works everywhere
select table_alias.column_alias
from (
select 1 column_alias from dual
) table_alias
但是如果我不能控制子选择怎么办？此外，某些RDBMS允许在为表别名时提供表别名和列别名：
But what if I don't have control over the subselect? Also, some RDBMS allow to provide both table AND column aliases when aliasing tables:
-- Seems to work in Postgres / SQL Server
select table_alias.column_alias
from (
select 1 from dual
) table_alias (column_alias)
但是某些RDBMS（例如MySQL）无法做到这一点。还有另一种方法吗？
But some RDBMS (e.g. MySQL) can't do that. Is there another way?
- 注意：这与任何特定的RDBMS无关，而与SQL有关
- 注意：我想省略星号，即没有
select *...
- Note: This isn't about any specific RDBMS, but just SQL in general
- Note: I'd like to omit the asterisk, i.e. no
select *...
这里是一个相关的问题：
A related question is this one here:
Is there a generic workaround to express a derived column list in Oracle (and MySQL)?
推荐答案
根据ANSI-92标准，它取决于实现。从第7.9节，第9.c节开始：
According to the ANSI-92 standard it is implementation dependent. From section 7.9, 9.c:
否则，第i列的<列名>与实现有关并且不同于SQL语句中包含的任何
引用的
表的
本身以外的任何列。 Otherwise, the <column name> of the i-th column of the is implementation-dependent and different from the <column name> of any column, other than itself, of a table referenced by any <table reference> contained in the SQL-statement.
换句话说，这都取决于您当时使用的RDBMS。
In other words, it's all going to depend on the RDBMS that you're using at the time.
顺便说一句，您可以查看 ANSI-92标准如果您正在寻找有趣的阅读内容。
BTW, you can check out the ANSI-92 standards if you're looking for some fun reading.
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