boolean_term(s) in SELECT clause

[Zarquan]

Email from Marco molinaro@oats.inaf.it and Sonia zorba@oats.inaf.it
2018-01-25

Working on TAP and ADQL in Trieste we (actually Sonia) found the following discrepancy/issue
ADQL seems to support a query like

SELECT "size"/2 AS half_size FROM TAP_SCHEMA.columns

while it seems not to support

SELECT "size" = 1 AS is_size_1 FROM TAP_SCHEMA.columns

At least Gregory's library (and TOPCAT, but IIRC TOPCAT uses Gregory's ADQL lib for validation) does so and it seems correct with respect to ADQL-2.0.

However in ADQL-2.1 the SELECT clause allows a <value_expression> that can be also a <boolean_value_expression>. This latter, however, contains no <boolean_term> and so disallows the above second query. That boolean_term would allow the SELECT to contain expressions like the above, exactly like it is in a WHERE clause (see <search_condition> definition).

We are wondering whether this is the intended behaviour or if it fits into the BNF-issues' list.
There's also the <boolean_function> BNF term (pg.56 in ADQL-2.1) that is blank, to complicate the view.

I'd like your opinion however, just to know if we can imagine to use boolean valued virtual columns (e.g.
like an a = value AS vcol) in the future or not (we already have possible alternatives).

[Zarquan]

You are right, <boolean_value_expression> probably should contain <boolean_term>.

I'll try to get this added to the specification when we edit it next.