Oracle 21c

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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, Oracle Database');

 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

WITH s AS
(
 SELECT DISTINCT student_id, REGEXP_SUBSTR(skills,'[^,]+',1,level) AS skills
   FROM student_skills
CONNECT BY level <= REGEXP_COUNT(skills,',')+1
    AND PRIOR student_id = student_id
    AND PRIOR sys_guid() IS NOT NULL 
)
SELECT student_id, LISTAGG(skills,', ') WITHIN GROUP (ORDER BY 0) AS skills
  FROM s
 GROUP BY student_id


STUDENT_ID
SKILLS


101
C,  Oracle Database,  SQL

102
CPP,  Java,  JavaScript, Java