SQL> create table students
  2  (
  3  sid number(3) primary key,
  4  name varchar2(20) not null,
  5  city varchar2(20),
  6  pincode number(6) unique
  7  );

insert into students values(&sid, '&name', '&city', &pincode);

CREATE TABLE exam (
    Eid NUMBER(3),
    Subname VARCHAR2(20),
    marks NUMBER(3),
    sid NUMBER(3),
    CONSTRAINT fk_sid FOREIGN KEY (sid) REFERENCES students(sid)
);

insert into exam values(&Eid, '&Subname', &marks, &sid);

 select * from students;

select * from exam;

select s.name, e.Subname, e.marks from students s join exam e on s.sid = e.sid where Eid = 101;

SELECT s.name FROM students s LEFT JOIN exam e ON s.sid = e.sid WHERE e.sid IS NULL;

SELECT e.Subname, COUNT(DISTINCT e.sid) AS total_students FROM exam e WHERE e.eid IN (101, 102) GROUP BY e.Subname;