Query creating XML returns empty tags when no data

Question

Hello everyone,
I have a problem that I just can't seem to solve.
I have the following query:
select
            xmlelement
               "users",
               xmlagg
                  xmlelement
                     "user",
                     xmlelement
                        "username",
                        e.pin
                     xmlagg
                        xmlelement
                           "details",
                           xmlforest
                              e.password as "password"
                             ,e.first_name as "forename"
                             ,e.surname as "surname"
                             ,'0' as "retired"
                             ,e.email_address as "email"
                             ,e.telephone_number as "phone"
                             ,'No External Ref' as "externalRef"
                             ,add_months(sysdate,+60) as "expiryDate"
            ) xml_out
         from   aqaost_examiners e
         group by e.pin;The query returns the XML in the desired structure.
But the problem I have is that when there is no data found by the query, I still get a single row returned with the following: <users></users>
I've tried so many things to try to have the query return nothing at all if there is no data to be found, but I just can't do it without something else going wrong (messing up the XML structure etc).
Please can someone help or point me in the right direction?
Thank you very much in advance!
Robin

Answer 1

odie_63 wrote:
Hi,
Peter,
It is the GROUP BY that does that.Actually no, the GROUP BY relates to the innermost XMLAgg and is mandatory if e.pin is not unique.
Leave it out, and you also seem to have one to many XMLAGG(?)That's assuming e.pin is unique in the table (which is probably a fair assumption in this case).
In fact, the behaviour comes from the topmost XMLAgg :
SQL> select xmlelement("users",
2           xmlagg(
3             xmlelement("user",
4               xmlelement("username", e.empno)
5             , xmlelement("details",
6                 xmlforest(
7                   e.ename as "name"
8                 , e.job as "job"
9                 )
10               )
11             )
12           )
13         ) xml_out
14  from scott.emp e
15  where 1 = 0
16  ;
XML_OUT
<users></users>As with other aggregate functions (like SUM or AVG) adding a GROUP BY 'something' solves the problem :
SQL> select xmlelement("users",
2           xmlagg(
3             xmlelement("user",
4               xmlelement("username", e.empno)
5             , xmlelement("details",
6                 xmlforest(
7                   e.ename as "name"
8                 , e.job as "job"
9                 )
10               )
11             )
12           )
13         ) xml_out
14  from scott.emp e
15  where 1 = 0
16  group by null
17  ;
no rows selectedRobin,
If the innermost XMLAgg is really necessary, then you can use a subquery :
select xmlelement("users", v.users)
from (
select xmlagg(
xmlelement("user",
xmlelement("username", e.empno)
, xmlagg(
xmlelement("details",
xmlforest(
e.ename as "name"
, e.job as "job"
) as users
from scott.emp e
group by e.empno
) v
where v.users is not null
Hello there,
Thank you very much! That sorted out the issue (adding the Group By Null)...it works a treat! I hadn't even considered that.
I didn't need the second XMLAGG in the end either, as I discovered when tinkering to try to sort the issue, but thank you for being so comprehensive with your reply.
Here is my finished result:
select xmlelement("users",
             xmlagg(
               xmlelement("user",
                 xmlelement("username", e.pin)
               , xmlelement("details",
                   xmlforest(
                              e.password as "password"
                             ,e.first_name as "forename"
                             ,e.surname as "surname"
                             ,'0' as "retired"
                             ,e.email_address as "email"
                             ,e.telephone_number as "phone"
                             ,'No External Ref' as "externalRef"
                             ,add_months(sysdate,+60) as "expiryDate"
          ).getclobval() xml_out
   from aqaost_examiners e
   group by null;Thank you so much again.
Robin