Column names for a table formed by a UNION

Question

Given a couple of simple tables like so:

create table R(foo text);
create table S(bar text);

If I were to union them together in a query, what do I call the column?

select T.????
from (
    select foo
    from R
    union
    select bar
    from S) as T;

Now, in mysql, I can apparently refer to the column of T as 'foo' -- the name of the matching column for the first relation in the union. In sqlite3, however, that doesn't seem to work. Is there a way to do it that's standard across all SQL implementations?

If not, how about just for sqlite3?

Correction: sqlite3 does allow you to refer to T's column as 'foo' after all! Oops!

Answer 1

Although there is no spelled rule, we can use the column names from the first subquery in the union query to fetch the union results.

Answer 2

Try to give an alias to columns;

select T.Col1
from (
    select foo as Col1
    from R
    union
    select bar as Col1
    from S) as T;

or If the name of column is not necessary then T.* will be enough.

Answer 3

you only need column aliases only in first select (tested in SQl Server 2008 R2)

select T.Col1
from (
    select 'val1' as Col1
    union
    select 'val2'
    union
    select 'val3'     
) as T;