[Ur] Subqueries.

[Alexei Golovko]

Thank you very match.
This seems to require also to optionify columns:
> table t : { X : option int, Y : option int }
> table s : { X : int, Y : int, Z : int }
> val test = dml (INSERT INTO t (X, Y) VALUES
>     ( (SELECT (s.X) FROM s WHERE s.Z = 1),
>     (SELECT (s.Y) FROM s WHERE s.Z = 1) )
> )
Is it possible to use similar query with
> table t' : { X : int, Y :  int }
(let it abort transaction instead inserting NULL)?

26.02.2012, 00:32, "Adam Chlipala" <adamc at impredicative.com>:

>
> The problem has to do with an aspect of the Ur/Web SQL encoding that
> isn't a literal transcription of standard SQL rules.  In particular,
> Ur/Web query results distinguish between columns pulled out of tables
> and columns computed from arbitrary expressions.  Your above subqueries
> generate output using the first kind of column, while Ur/Web expects the
> second kind of column.  A subquery should return exactly one column per
> row, where that column is an "arbitrary computed" column with any name.
>
> It turns out you can fix the example by putting the selected item in
> parens, e.g.:
>      SELECT (s.X) FROM ...
> which "demotes" that output column to "arbitrary computed" status,
> forgetting that it comes from a table, and giving it name [1] by default.
>
> The Ur type system doesn't provide an obvious way to type the subquery
> operator where it would work with either kind of output column.