Op 3 feb 2013, om 02:59 heeft Igor Tandetnik het volgende geschreven:
On 2/2/2013 6:46 PM, Gabor Grothendieck wrote:
In 3.7.11 there was a change to support the feature in the subject
which refers to guaranteeing that y comes from the same row having
maximum x.. See:
http://pages.citebite.com/o9y9n0p9neyt
Did this or other change also enhance the having clause to add a
feature to support a query containing "having max(...)" such as the
query here:
http://r.789695.n4.nabble.com/Filter-according-to-the-latest-data-tp4657248p4657305.html
The query in that thread is of the form "select * from MyTable group
by Name having max(Timestamp)", and the expectation, somehow, is
that the HAVING clause would cause each group to be represented by a
row for which max(Timestamp) is reached. I'm not sure where this
expectation comes from. This is a valid SQL statement whose HAVING
clause means "only include a group in the resultset if
max(Timestamp) for this group is logically true" (that is, not NULL,
0, empty string or empty blob).
A semantic change of the nature you envision is not backward
compatible - it modifies the meaning of existing valid statements.
Also, I'm pretty sure it's not supported by any SQL standard; and
I'm not aware of any DBMS that would interpret the statement the way
you want (which doesn't mean none such exists, of course).
All in all, It seems unlikely that such a proposal would be
entertained.
--
Igor Tandetnik
Hi,
Is it then not a perfect solution? it works, if in the example the
timestamp is always logically true (i.e nit 0 or NULL). Otherwise you
might write something like HAVING MAX(timestamp) OR MAX(timestamp) IS
NULL.
This construction comes in useful to deal with the issue as was
recently observed with this featurre (see
http://www.mail-archive.com/sqlite-users@sqlite.org/msg74761.html
The construction does not involve any semantic change. The question is
still if it will work also in future versions of SQLite.
Regards, EPasma
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