问题
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
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
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)
But some RDBMS (e.g. MySQL) can't do that. Is there another way?
- 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 *...
A related question is this one here:
Is there a generic workaround to express a derived column list in Oracle (and MySQL)?
回答1:
According to the ANSI-92 standard it is implementation dependent. From section 7.9, 9.c:
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.
In other words, it's all going to depend on the RDBMS that you're using at the time.
BTW, you can check out the ANSI-92 standards if you're looking for some fun reading.
回答2:
In Oracle, you can do
select aa."2" from ( select 2 from dual ) aa
来源：https://stackoverflow.com/questions/6669088/how-to-select-an-unaliased-numeric-literal-from-a-sub-select