James K. Lowden wrote:
> 1. Last I checked, SELECT in a column position in the SELECT clause as
> in
>
> select foo (select ...)
>
> is not permitted by the SQL standard.
This example indeed is not valid SQL syntax.
However, SELECT in a column position is allowed:
select (select 42);
This is called a scalar subquery, and it's allowed in place of any
expression. (The SQL standard requires it to return exactly one row.)
> 1. Is the left-join on a CTE apt to be more effecient than the version
> that uses a correlated subquery in the SELECT clause?
Some databases' query optimizers can do a better job with joins than
with subqueries. In SQLite, left joins and correlated subqueries
are executed in exactly the same way.
> 2. Is there any performance difference between
>
> sum(ca1 +ca2 + exam)
> and
> sum(ca1) + sum(ca2) + sum(exam)
The second one will do three aggregations in parallel, but I'd expect
the difference to be too small to be noticeable.
Regards,
Clemens
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