Oracle 18c

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CREATE TABLE test_data ( Student_Id, Student_Name, Maths, English, History, Physics ) AS
SELECT 1, 'ABC', 93, 89, 90, 70 FROM DUAL UNION ALL
SELECT 2, 'XYZ', 88, 98, 88, 80 FROM DUAL UNION ALL
SELECT 3, 'DEF', 79, 78, 87, 90 FROM DUAL;
3 rows affected

SELECT score
FROM   test_data
UNPIVOT ( score FOR subject IN ( Maths, English, History, Physics ) )
WHERE subject      = 'MATHS'
AND   student_name = 'ABC'


SCORE


93


CREATE FUNCTION getScore(
  i_student_name IN TEST_DATA.STUDENT_NAME%TYPE,
  i_subject      IN VARCHAR2
) RETURN NUMBER
IS
  p_score NUMBER(3,0);
BEGIN
  SELECT CASE i_subject
         WHEN 'MATHS' THEN Maths
         WHEN 'ENGLISH' THEN English
         WHEN 'HISTORY' THEN History
         WHEN 'PHYSICS' THEN Physics
         END
  INTO p_score
  FROM   test_data
  WHERE  student_name = i_student_name;
  
  RETURN p_score;
EXCEPTION
  WHEN NO_DATA_FOUND THEN
    RETURN NULL;
END;
/
SELECT getScore( 'ABC', 'MATHS' ) FROM DUAL;


GETSCORE('ABC','MATHS')


93