1.select avg(mark.english) from cust left join mark on cust.studentno = mark.studentno;
2.select mark.math, cust.name,cust.address, cust.address from cust left join mark on cust.studentno = mark.studentno where cust.studentno in (11,22,33,44,55);
3.select cust.name, mark.computer from cust left join mark on cust.studentno = mark.studentno order by mark.computer desc;
4.select cust.studentno, cust.name, (mark.english + mark.math + mark.computer) total from cust left join mark on cust.studentno = mark.studentno where (mark.english + mark.math + mark.computer) > 240 order by total desc;