Re: [INTERFACES] JDBC next() method - Mailing list pgsql-interfaces

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
argumentsource of each such <set function specification> and the result of the <query specification> is a table con-
sistingof 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-thcolumn 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
ofthe i-th <value expression>. When a <value expression> is applied to a given group of T, that group is the argument
orargument 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