Writing user defined function the same way as the oracle functions

Question

Hi Guys,
In one of the interviews , I have attended the guy asked me to write a user defined function which will take the column name and list all the values .
For example
Table Name:Employees
Column Name: Employee_Name
Employee_name
Scott
Ivgun
Jack
Shane
The query should be in this fromat
SELECT col_agg(Employee_name) emp_name from Employees;
The output shoulld be
Emp_names
scott,ivgun,jack,shane
Please let me know if this is possible
Any suggestions will be highly appreciated.
Thanks,
Ranjan

Answer 1

You could certainly write a user defined aggregate function if you like...
e.g. this function will aggregate strings into a CLOB...
create or replace type clobagg_type as object
  text clob,
  static function ODCIAggregateInitialize(sctx in out clobagg_type) return number,
  member function ODCIAggregateIterate(self in out clobagg_type, value in clob) return number,
  member function ODCIAggregateTerminate(self in clobagg_type, returnvalue out clob, flags in number) return number,
  member function ODCIAggregateMerge(self in out clobagg_type, ctx2 in clobagg_type) return number
create or replace type body clobagg_type is
  static function ODCIAggregateInitialize(sctx in out clobagg_type) return number is
  begin
    sctx := clobagg_type(null) ;
    return ODCIConst.Success ;
  end;
  member function ODCIAggregateIterate(self in out clobagg_type, value in clob) return number is
  begin
    self.text := self.text || value ;
    return ODCIConst.Success;
  end;
  member function ODCIAggregateTerminate(self in clobagg_type, returnvalue out clob, flags in number) return number is
  begin
    returnValue := self.text;
    return ODCIConst.Success;
  end;
  member function ODCIAggregateMerge(self in out clobagg_type, ctx2 in clobagg_type) return number is
  begin
    self.text := self.text || ctx2.text;
    return ODCIConst.Success;
  end;
end;
create or replace function clobagg(input clob) return clob
  deterministic
  parallel_enable
  aggregate using clobagg_type;
SQL> select trim(',' from clobagg(ename||',')) as enames from emp;
ENAMES
SMITH,ALLEN,WARD,JONES,MARTIN,BLAKE,CLARK,SCOTT,KING,TURNER,ADAMS,JAMES,FORD,MILLER
SQL> ed
Wrote file afiedt.buf
  1  with t as
  2    (select 'PFL' c1, 0 c2,110 c3 from dual union all
  3     select 'LHL', 0 ,111 from dual union all
  4     select 'PHL', 1, 111 from dual union all
  5     select 'CHL', 2, 111 from dual union all
  6     select 'DHL', 0, 112 from dual union all
  7     select 'VHL', 1, 112 from dual union all
  8     select 'CPHL', 0, 114 from dual union all
  9     select 'WDCL', 1, 114 from dual union all
10     select 'AHL' ,2 ,114 from dual union all
11     select 'NFDL', 3, 114 from dual)
12  --
13  -- end of test data
14  --
15  select trim(clobagg(c1||' ')) as c1, c3
16  from (select * from t order by c3, c2)
17  group by c3
18* order by c3
SQL> /
C1                                     C3
PFL                                   110
LHL CHL PHL                           111
DHL VHL                               112
CPHL AHL NFDL WDCL                    114Ok, it's more than just a function as it uses an object type linking into the internals of the ODCI... but it does what you ask. :)