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

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_nam         


        

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.

Answer 2

I assume you're talking about the CREATE TABLE ... AS SELECT command. This looks like an SQL extension to me.

Standard SQL does not allow you to use the same column name for different columns, and SQLite appears to be allowing that in its extension, but working around it. While a simple, naked select statement simply uses as to set the column name, create table ... as select uses it to create a brand new table with those column names.

As an aside, it would be interesting to see what the naked select does when you try to use the duplicated column, such as in an order by clause.

If you were allowed to have multiple columns with the same name, it would be a little difficult for the execution engine to figure out what you meant with:

select overloaded_name from table;

The reason why you can do it in the select is to allow things like:

select id, surname   as name from users where surname is not null
union all
select id, firstname as name from users where surname is     null

so that you end up with a single name column.

As to whether there's a good reason, SQLite is probably assuming you know what you're doing when you specify the same column name for two different columns. Its essence seems to be to allow a great deal of latitude to the user (as evidenced by the fact that the columns are dynamically typed, for example).

The alternative would be to simply refuse your request, which is what I'd prefer, but the developers of SQLite are probably more liberal (or less anal-retentive) than I :-)

Answer 3

The table itself can't have duplicate column names because inserting and updating would be messed up. Which column gets the data?

During selects the "duplicates" are just column labels so do not hurt anything.