Count(1) returns null in group by

Question

hi gems..good afternoon..
I read that the COUNT() function always returns 0 (zero) if there is no matching rows in the table.
The following code returns the 0 as expected:
SELECT COUNT(1) FROM book_table
             WHERE client_id = 10009
               AND book_id = 5465465
               AND book_sub_id = 'gfdf'
               AND amount = 78686But when I used the GROUP BY clause with the query, then it returned nothing:
SELECT COUNT(1) FROM book_table
             WHERE client_id = 10009
               AND book_id = 5465465
               AND book_sub_id = 'gfdf'
               AND amount = 78686
group by client_id,book_id,book_sub_id,amountWhy this is happening..please suggest...

Answer 1

gogol wrote:
But Ranit...
Again I am thinking...the COUNT() is an aggregate function. Now a function should return something(as per my plsql knowledge) and in this case the return datatype is integer. So why isnt it returning zero..Don't think like that sandy.
The Group By is actually done on an empty result set, so the result is neither 0 nor NULL
It is an empty result set.
Check this -- http://stackoverflow.com/questions/2552086/does-count-always-return-a-result
>
The "return value of the 'count' function" is ALWAYS a non-null integer, without exception. By mentioning "group by", you're referencing the containing query and changing the subject of "return value" from "count function" to "query's result set". A non-grouped count query produces a result set of a single record containing the return value of count. Alternatively, a grouped count query produces a result set where each record contains a count value. In that case, if there are no groups for count to run on, count is never run and the "query return value" is an empty set.
>
Hope this Helps.
Ranit B.
Edited by: ranit B on Nov 23, 2012 5:25 PM