create table student (stuid number(6) primary key,name varchar2(30),mob varchar2(20));

insert into student (1,'abc','92333444444');
insert into student (2,'aac','92333004444');
insert into student (3,'avc','92333004444');
insert into student (4,'afc','92333233444');
insert into student (5,'atc','92333233444');
insert into student (6,'arc','92333444444');
insert into student (7,'aec','92333444444');
insert into student (8,'atc','92333123444');
insert into student (9,'arc','92332334444');
insert into student (10,'aec','92333444444');

i want to get the result as:

mob             Stuid
---------------------------------------------------
92333444444     (1,6,7,10)
92333004444     (2,3)
92332334444     (4,5,9)

SQL> insert into student (1,'abc','92333444444');
insert into student (1,'abc','92333444444')
                     *
ERROR at line 1:
ORA-00928: missing SELECT keyword

SQL> select mob,
  6  /
MOB                  STUID
-------------------- ------------------------------
92332334444          (9)
92333004444          (2,3)
92333123444          (8)
92333233444          (4,5)
92333444444          (1,6,7,10)

create table student (stuid number(6) primary key,name varchar2(30),mob varchar2(20));

insert into student values (1,'abc','92333444444');
insert into student values (2,'aac','92333004444');
insert into student values (3,'avc','92333004444');
insert into student values (4,'afc','92333233444');
insert into student values (5,'atc','92333233444');
insert into student values (6,'arc','92333444444');
insert into student values (7,'aec','92333444444');
insert into student values (8,'atc','92333123444');
insert into student values (9,'arc','92332334444');
insert into student values (10,'aec','92333444444');

i want to get the result as:

mob             Stuid
---------------------------------------------------
92333444444     (1,6,7,10)
92333004444     (2,3)
92332334444     (4,5,9)

SQL> select
  2  p.mob,
  3      (select xmlagg(xmlelement(E, d.stuid || ',')).extract('//text()') AS SE
 from student d where d.mob = p.mob)
  4  from student p;

ERROR:
ORA-22922: nonexistent LOB value

MOB	            SEE
923056733136	[datatype]
923042039519	[datatype]
923063375586	[datatype]
923063375586	[datatype]
923337663357	[datatype]
923337663357	[datatype]
923016301936	[datatype]

SQL> select * From student;

       STUID NAME                           MOB
------------ ------------------------------ --------------------
           1 abc                            92333444444
           2 aac                            92333004444
           3 avc                            92333004444
           4 afc                            92333233444
           5 atc                            92333233444
           6 arc                            92333444444
           7 aec                            92333444444
           8 atc                            92333123444
           9 arc                            92332334444
          10 aec                            92333444444

10 rows selected.

SQL> create or replace function f_mob (par_mob in varchar2)
  2    return varchar2
  3  is
  4    retval varchar2(30);
  5  begin
  6    for cur_r in (select stuid from student
  7                  where mob = par_mob)
  8    loop
  9      retval := retval ||','|| cur_r.stuid;
 10    end loop;
 11
 12    retval := ltrim(rtrim(retval, ','), ',');
 13    return retval;
 14  end;
 15  /

Function created.

SQL> select f_mob('92333004444') one,
  2         f_mob('92333444444') two
  3  from dual;

ONE                  TWO
-------------------- --------------------
2,3                  1,6,7,10

SQL>

CREATE OR REPLACE TYPE t_string_agg AS OBJECT
(
  g_string  VARCHAR2(32767),

  STATIC FUNCTION ODCIAggregateInitialize(sctx  IN OUT  t_string_agg)
    RETURN NUMBER,

  MEMBER FUNCTION ODCIAggregateIterate(self   IN OUT  t_string_agg,
                                       value  IN      VARCHAR2 )
     RETURN NUMBER,

  MEMBER FUNCTION ODCIAggregateTerminate(self         IN   t_string_agg,
                                         returnValue  OUT  VARCHAR2,
                                         flags        IN   NUMBER)
    RETURN NUMBER,

  MEMBER FUNCTION ODCIAggregateMerge(self  IN OUT  t_string_agg,
                                     ctx2  IN      t_string_agg)
    RETURN NUMBER
);
/
SHOW ERRORS


CREATE OR REPLACE TYPE BODY t_string_agg IS
  STATIC FUNCTION ODCIAggregateInitialize(sctx  IN OUT  t_string_agg)
    RETURN NUMBER IS
  BEGIN
    sctx := t_string_agg(NULL);
    RETURN ODCIConst.Success;
  END;

  MEMBER FUNCTION ODCIAggregateIterate(self   IN OUT  t_string_agg,
                                       value  IN      VARCHAR2 )
    RETURN NUMBER IS
  BEGIN
    SELF.g_string := self.g_string || ',' || value;
    RETURN ODCIConst.Success;
  END;

  MEMBER FUNCTION ODCIAggregateTerminate(self         IN   t_string_agg,
                                         returnValue  OUT  VARCHAR2,
                                         flags        IN   NUMBER)
    RETURN NUMBER IS
  BEGIN
    returnValue := SUBSTR(SELF.g_string, 2);
    RETURN ODCIConst.Success;
  END;

  MEMBER FUNCTION ODCIAggregateMerge(self  IN OUT  t_string_agg,
                                     ctx2  IN      t_string_agg)
    RETURN NUMBER IS
  BEGIN
    SELF.g_string := SELF.g_string || ctx2.g_string;
    RETURN ODCIConst.Success;
  END;
END;
/
SHOW ERRORS


CREATE OR REPLACE FUNCTION string_agg (p_input VARCHAR2)
RETURN VARCHAR2
PARALLEL_ENABLE AGGREGATE USING t_string_agg;
/
SHOW ERRORS
The aggregate function is implemented using a type and type body, and is used within a query.

COLUMN employees FORMAT A50

SELECT /*+ PARALLEL(2) */ deptno, string_agg(ename) AS employees
FROM   emp
GROUP BY deptno;