Oracle 18c

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create table student_skills
(STUDENT_ID number(10),
SEMESTER varchar2(5),
SKILLS varchar2(50));
BEGIN
insert into student_skills (STUDENT_ID, SEMESTER, SKILLS) values (101, '1', 'C, SQL');
insert into student_skills (STUDENT_ID, SEMESTER, SKILLS) values (101, '2', 'C, CPP');
insert into student_skills (STUDENT_ID, SEMESTER, SKILLS) values (102, '1', 'CPP, Java');
insert into student_skills (STUDENT_ID, SEMESTER, SKILLS) values (102, '2', 'Java, JavaScript');
END;
/
1 rows affected

SELECT student_id,
       XMLQUERY(
         'string-join(distinct-values(//text()), ", ")'
         PASSING XMLTYPE(skills)
         RETURNING CONTENT
       ).getStringVal() AS skills
FROM   (
  SELECT student_id,
         '<b>'
         || LISTAGG('<a>' || REPLACE(skills, ', ', '</a><a>') || '</a>')
              WITHIN GROUP (ORDER BY semester)
         || '</b>' AS skills
  FROM   student_skills
  GROUP BY
         student_id
);


STUDENT_ID
SKILLS


101
C, SQL, CPP

102
CPP, Java, JavaScript