Oh, I see, thanks for your quick reply.
On Fri, Dec 28, 2012 at 3:47 PM, Jov <zhao6...@gmail.com> wrote:
>
>
> 2012/12/28 wd <w...@wdicc.com>
>
>> hi,
>>
>> wd_test=# \d t1
>> Table "public.t1"
>> Column | Type | Modifiers
>> --------+---------+-------------------------------------------------
>> id | integer | not null default nextval('t1_id_seq'::regclass)
>> tag | text |
>>
>> wd_test=# select * from t1;
>> id | tag
>> ----+-----
>> 1 | a
>> 2 | a
>> 3 | b
>> 4 | c
>> 5 | b
>> (5 rows)
>>
>> ---- this sql will not group by the case result.
>> wd_test=# select case t1.tag when 'a' then '1' else '0' end as tag,
>> count(*) from t1 group by tag;
>>
>
> here the group by key tag is t1.tag,not the tag int the select list
>
>
>> tag | count
>> -----+-------
>> 0 | 1
>> 0 | 2
>> 1 | 2
>> (3 rows)
>>
>> ---- this sql will group by the case result.
>> wd_test=# select case t1.tag when 'a' then '1' else '0' end as ttag,
>> count(*) from t1 group by ttag;
>>
> here the ttag is the select list ttag,it is equal with group by 1.
>
>
>> ttag | count
>> ------+-------
>> 0 | 3
>> 1 | 2
>> (2 rows)
>>
>
> http://www.postgresql.org/docs/9.2/static/sql-select.html#SQL-GROUPBY
>
>
>> GROUP BY will condense into a single row all selected rows that share
>> the same values for the grouped expressions. expression can be an input
>> column name, or the name or ordinal number of an output column (SELECT list
>> item), or an arbitrary expression formed from input-column values. *In
>> case of ambiguity, a GROUP BY name will be interpreted as an
>> input-column name rather than an output column name.*
>
>
> so it is not a bug.
>
