>>>>> "Hitoshi" == Hitoshi Harada <umi.tan...@gmail.com> writes:
Hitoshi> Hi, During reviewing aggregates ORDER BY, I was reading spec
Hitoshi> and found description like:
Hitoshi> == snip ==
Hitoshi> Of the rows in the aggregation, the following do not qualify:
Hitoshi> — If DISTINCT is specified, then redundant duplicates.
Hitoshi> — Every row in which the <value expression> evaluates to the null
value.
Hitoshi> == /snip ==
Where did you find that?
The SQL2008 last-call draft says this:
4) If <general set function> is specified, then:
a) Let TX be the single-column table that is the result of applying
the <value expression> to each row of T1 and eliminating null
values. If one or more null values are eliminated, then a
completion condition is raised: warning -- null value eliminated
in set function.
b) Case:
i) If DISTINCT is specified, then let TXA be the result of
eliminating redundant duplicate values from TX, using the
comparison rules specified in Subclause 8.2, "<comparison
predicate>", to identify the redundant duplicate values.
ii) Otherwise, let TXA be TX.
[more subclauses of rule (4) snipped as irrelevant]
8) If <array aggregate function> is specified, then:
a) If <sort specification list> is specified, then let K be the
number of <sort key>s; otherwise, let K be 0 (zero).
b) Let TXA be the table of K+1 columns obtained by applying the
<value expression> immediately contained in the <array aggregate
function> to each row of T1 to obtain the first column of TXA,
and, for all i, 1 (one) i K, applying the <value expression>
simply contained in the i-th <sort key> to each row of T1 to
obtain the (i+1)-th column of TXA.
c) Let TXA be ordered according to the values of the <sort key>s
found in the second through (K+1)-th columns of TXA. If K is 0
(zero), then the ordering of TXA is implementation-dependent.
d) Let N be the number of rows in TXA.
e) If N is greater than IDMC, then an exception condition is
raised: data exception -- array data, right truncation.
f) Let Ri, 1 (one) i N, be the rows of TXA according to the
ordering of TXA.
g) Case:
i) If TXA is empty, then the result of <array aggregate
function> is the null value.
ii) Otherwise, the result of <array aggregate function> is an
array of N elements such that for all i, 1 (one) i N, the
value of the i-th element is the value of the first column
of Ri.
NOTE 267 -- Null values are not eliminated when computing <array
aggregate function>. This, plus the optional <sort specification
list>, sets <array aggregate function> apart from <general set
function>s.
array_agg is an <array aggregate function> (in fact the only such),
whereas <general set function> includes almost all the other single-arg
aggregates (avg, min, max, etc.)
--
Andrew (irc:RhodiumToad)
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