Question

In sqlite3, I can force two columns to alias to the same name, as in the following query:

SELECT field_one AS overloaded_name,
       field_two AS overloaded_name
FROM my_table;

It returns the following:

overloaded_name  overloaded_name
---------------  ---------------
1                2
3                4
...              ...

… and so on.

However, if I create a named table using the same syntax, it appends one of the aliases with a :1:

sqlite> CREATE TABLE temp AS
          SELECT field_one AS overloaded_name,
                 field_two AS overloaded_name
          FROM my_table;
sqlite> .schema temp
CREATE TABLE temp(
  overloaded_name TEXT,
  "overloaded_name:1" TEXT
);

I ran the original query just to see if this was possible, and I was surprised that it was allowed. Is there any good reason to do this? Assuming there isn’t, why is this allowed at all?

EDIT:

I should clarify: the question is twofold: why is the table creation allowed to succeed, and (more importantly) why is the original select allowed in the first place?

Also, see my clarification above with respect to table creation.

Answer 1

I can force two columns to alias to the same name…
why is [this] allowed in the first place?

This can be attributed to the shackles of compatibility. In the SQL Standards, nothing is ever deprecated. An early version of the Standard allowed the result of a table expression to include columns with duplicate names, probably because an influential vendor had allowed it, possibly due to the inclusion of a bug or the omission of a design feature, and weren’t prepared to take the risk of breaking their customers’ code (the shackles of compatibility again).

Is there any use to duplicate column names in a table?

In the relational model, every attribute of every relation has a name that is unique within the relevant relation. Just because SQL allows duplicate column names that doesn’t mean that as a SQL coder you should utilise such as feature; in fact I’d say you have to vigilant not to invoke this feature in error. I can’t think of any good reason to have duplicate column names in a table but I can think of many obvious bad ones. Such a table would not be a relation and that can’t be a good thing!

why is the [base] table creation allowed to succeed

Undoubtedly an ‘extension’ to (a.k.a purposeful violation of) the SQL Standards, I suppose it could be perceived as a reasonable feature: if I attempt to create columns with duplicate names the system automatically disambigutes them by suffixing an ordinal number. In fact, the SQL Standard specifies that there be an implementation dependent way to ensure the result of a table expression does not implicitly have duplicate column names (but as you point out in the question this does not perclude the user from explicitly using duplicate AS clauses). However, I personally think the Standard behaviour of disallowing the duplicate name and raising an error is the correct one. Aside from the above reasons (i.e. that duplicate columns in the same table are of no good use), a SQL script that creates an object without knowing if the system has honoured that name will be error prone.