Quite possibly my foot will soon be in my mouth, but in the PFD 14.6.9,
the target expression for count must be "this" or a variable name (well,
it says "can" -- am I reading this incorrectly?). If this is truly a
restriction, I do not see the relevance of this addition, which only
makes sense when talking about counting field expressions or the results
of calculations.
An edge case to consider: If null values are allowed as primary keys in
an object using application identity, the likely SQL "select
count(primary_key_column) from my_table" may incorrectly omit a valid
instance, meaning that "SELECT COUNT(this) FROM MyClass" returns a
different value than Collection.size() when invoked on "SELECT this FROM
MyClass". Acceptable?
Wes
Craig L Russell wrote:
Javadogs,
Please comment if you have any issues with the proposal.
Issue 143
H
Treatment of null values in JDOQL COUNT
JDOQL currently says nothing about the treatment of null values in the
COUNT clause of a query. Based on the SQL treatment, and the fact that
JDOQL is intended to be executed by the back end datastore (see below)
I propose adding this to the JDOQL chapter:
<proposed>
If null values are aggregated, they do not participate in the
aggregate result. If all of the expressions to be aggregated evaluate
to null, the result is the same as if there were no instances that
match the filter.
</proposed>
The behavior of aggregates is described in Part 2 of the ANSI spec,
section 10.9.
1) A null column is excluded from a COUNT( colName ) aggregate. This
is described in the section 10.9 under General Rules 4a. The database
is supposed to raise a warning: "warning--null value eliminated in set
function"
2) Unless you specify the DISTINCT keyword, the COUNT aggregate will
not filter out duplicates. Each row, regardless of whether it is a
duplicate, will go into the tally. This is described in the same
section underGeneral Rules 4b.
For the record, Derby exhibits this ANSI behavior. To summarize:
-- the following query eliminates rows with null in column "a"
select count( a ) from foo;
-- the following query eliminates rows with null in column "a"
-- and eliminates duplicates
select count( distinct a ) from foo;
Craig Russell
Architect, Sun Java Enterprise System http://java.sun.com/products/jdo
408 276-5638 mailto:[EMAIL PROTECTED]
P.S. A good JDO? O, Gasp!
