Re: [INTERFACES] JDBC next() method

At 19:40 +0300 on 25/04/1999, Tom Lane wrote:

> Well, this is certainly adequate precedent for the behavior of these
> particular aggregates --- although I'd have to say that the standard-
> writers blew it for SUM; SUM of an empty set ought to return 0 not
> null. (It looks like Postgres follows the spec, however.)

I don't agree. Suppose you want to sum of all your banking transactions in
January. There is a distinction between getting a 0, meaning you had a
balanced budget in January, and getting a null, meaning you made no
transactions in January.

> Now that I think about it, the arguments on the hackers list were not
> about the plain SELECT case but about the GROUP BY case. For example,
> if you do
>
> SELECT productname, AVG(saleprice) FROM sales GROUP BY productname;
>
> then you get a row in the output for each different productname, and
> a separate instance of AVG is run over the prices for each group.
> (Unless there are NULLs in the saleprice column, none of the AVG
> instances could ever return a null result.)
>
> BUT: what happens if the sales table is empty? There are no
> productnames, therefore no groups, therefore no rows ought to appear
> in the output (IMHO). However, what Postgres actually does right now
> is to emit one all-nulls row (but only if an aggregate function was
> used; if you say "SELECT productname FROM sales GROUP BY productname"
> then you get no rows). That is the behavior that we've gone 'round and
> 'round on without any resolution; it seems obviously inconsistent to me,
> but others think it's OK because it parallels what happens in the non-
> GROUP BY case.
>
> Is there anything in the SQL92 spec addressing this point?

Ooh, definitely... Here are the general rules regarding a query expression
(i.e. a general select statement). A grouped table is defined somewhere
else in the document as the result of a group by or having clause. You
really should read the definitions (I can make my draft available on the
web for a while. I don't remember where I downloaded it).

<<< Begin quotation >>>

General Rules

1) Case:

a) If T is not a grouped table, then

Case:

i) If the <select list> contains a <set function specifica-
tion> that contains a reference to a column of T or di-
rectly contains a <set function specification> that does
not contain an outer reference, then T is the argument or
argument source of each such <set function specification>
and the result of the <query specification> is a table con-
sisting of 1 row. The i-th value of the row is the value
specified by the i-th <value expression>.

ii) If the <select list> does not include a <set function spec-
ification> that contains a reference to T, then each <value
expression> is applied to each row of T yielding a table of
M rows, where M is the cardinality of T. The i-th column of
the table contains the values derived by the evaluation of
the i-th <value expression>.

Case:

1) If the <set quantifier> DISTINCT is not specified, then
the table is the result of the <query specification>.

2) If the <set quantifier> DISTINCT is specified, then the
result of the <query specification> is the table derived
from that table by the elimination of any redundant
duplicate rows.

b) If T is a grouped table, then

Case:

i) If T has 0 groups, then the result of the <query specifica-
tion> is an empty table.

ii) If T has one or more groups, then each <value expression>
is applied to each group of T yielding a table of M rows,
where M is the number of groups in T. The i-th column of
the table contains the values derived by the evaluation of
the i-th <value expression>. When a <value expression> is
applied to a given group of T, that group is the argument
or argument source of each <set function specification> in
the <value expression>.

Case:

1) If the <set quantifier> DISTINCT is not specified, then
the table is the result of the <query specification>.

2) If the <set quantifier> DISTINCT is specified, then the
result of the <query specification> is the table derived
from T by the elimination of any redundant duplicate
rows.

<<< End quotation >>>

Which implies exactly what you say. Perhaps you should forward this to the
Hackers list to re-start the argument - I am not a subscriber there.

Herouth

--
Herouth Maoz, Internet developer.
Open University of Israel - Telem project
http://telem.openu.ac.il/~herutma