Conditional SQL?

Question

Is there a way to write a piece of pure SQL (no PL/SQL allowed!) which will conditionally execute a query or not.
Something like
select case when count(*) = 1 then (select * from dba_objects) else null end from my_table;...that is, count the records in MY_TABLE, and if there is one record in there, then select from dba_objects. If there are 0 or 2 or more records, then don't do anything.
Obviously this is easy to do in PL/SQL... but I would like to know if it is at all possible in SQL alone (let's assume version 10 upwards)
If there isn't, then that's fine as an answer, too!

Answer 1

Perhaps doing it along these lines will do what you want (I've done the conditional bit slightly different for demonstration purposes)...
SQL> with t as (select case when count(*) = &cnt_chk then
  2                      xmltype(dbms_xmlgen.getxml('select empno
  3                                                        ,ename
  4                                                        ,job
  5                                                        ,mgr
  6                                                        ,to_char(hiredate,''YYYY-MM-DD'') as hiredate
  7                                                        ,sal
  8                                                        ,comm
  9                                                        ,deptno
10                                                  from emp'
11                                                 )
12                             )
13                    else
14                      xmltype('<ROWSET/>')
15                    end as xml
16             from dual
17            )
18  select x.*
19  from   t
20        ,xmltable('/ROWSET/ROW'
21                  passing t.xml
22                  columns empno    number       path '/ROW/EMPNO'
23                         ,ename    varchar2(10) path '/ROW/ENAME'
24                         ,job      varchar2(9)  path '/ROW/JOB'
25                         ,mgr      number       path '/ROW/MGR'
26                         ,hiredate date         path '/ROW/HIREDATE'
27                         ,sal      number       path '/ROW/SAL'
28                         ,comm     number       path '/ROW/COMM'
29                         ,deptno   number       path '/ROW/DEPTNO'
30                 ) x
31  /
Enter value for cnt_chk: 1
old   1: with t as (select case when count(*) = &cnt_chk then
new   1: with t as (select case when count(*) = 1 then
     EMPNO ENAME      JOB              MGR HIREDATE           SAL       COMM     DEPTNO
      7369 SMITH      CLERK           7902 17-DEC-1980        800                    20
      7499 ALLEN      SALESMAN        7698 20-FEB-1981       1600        300         30
      7521 WARD       SALESMAN        7698 22-FEB-1981       1250        500         30
      7566 JONES      MANAGER         7839 02-APR-1981       2975                    20
      7654 MARTIN     SALESMAN        7698 28-SEP-1981       1250       1400         30
      7698 BLAKE      MANAGER         7839 01-MAY-1981       2850                    30
      7782 CLARK      MANAGER         7839 09-JUN-1981       2450                    10
      7788 SCOTT      ANALYST         7566 19-APR-1987       3000                    20
      7839 KING       PRESIDENT            17-NOV-1981       5000                    10
      7844 TURNER     SALESMAN        7698 08-SEP-1981       1500          0         30
      7876 ADAMS      CLERK           7788 23-MAY-1987       1100                    20
      7900 JAMES      CLERK           7698 03-DEC-1981        950                    30
      7902 FORD       ANALYST         7566 03-DEC-1981       3000                    20
      7934 MILLER     CLERK           7782 23-JAN-1982       1300                    10
14 rows selected.
SQL> /
Enter value for cnt_chk: 0
old   1: with t as (select case when count(*) = &cnt_chk then
new   1: with t as (select case when count(*) = 0 then
no rows selectedAs far as I can tell, the query to get the data (in my example from the emp table) is only executed by the XMLGen.GetXML if the condition matches, the only thing being that you're left with XML data which you then need to extract back out to flat data.