Queries in SQL Oracle

These questions are the most frequently asked in interviews.

1. To fetch ALTERNATE records from a table. (EVEN NUMBERED)
   select * from emp where rowid in (select decode(mod(rownum,2),0,rowid, null) from emp);
2. To select ALTERNATE records from a table. (ODD NUMBERED)
   select * from emp where rowid in (select decode(mod(rownum,2),0,null ,rowid) from emp);
3. Find the 3rd MAX salary in the emp table.
   select distinct sal from emp e1 where 3 = (select count(distinct sal) from emp e2 where e1.sal <= e2.sal);
4. Find the 3rd MIN salary in the emp table.
   select distinct sal from emp e1 where 3 = (select count(distinct sal) from emp e2where e1.sal >= e2.sal);
5. Select FIRST n records from a table.
   select * from emp where rownum <= &n;
6. Select LAST n records from a table
   select * from emp minus select * from emp where rownum <= (select count(*) - &n from emp);
7. List dept no., Dept name for all the departments in which there are no employees in the department.
   select * from dept where deptno not in (select deptno from emp);
   alternate solution: select * from dept a where not exists (select * from emp b where a.deptno = b.deptno);
   altertnate solution: select empno,ename,b.deptno,dname from emp a, dept b where a.deptno(+) = b.deptno and empno is null;
8. How to get 3 Max salaries ?
   select distinct sal from emp a where 3 >= (select count(distinct sal) from emp b where a.sal <= b.sal) order by a.sal desc;
9. How to get 3 Min salaries ?
   select distinct sal from emp a where 3 >= (select count(distinct sal) from emp b where a.sal >= b.sal);
10. How to get nth max salaries ?
    select distinct hiredate from emp a where &n = (select count(distinct sal) from emp b where a.sal >= b.sal);
11. Select DISTINCT RECORDS from emp table.
    select * from emp a where rowid = (select max(rowid) from emp b where a.empno=b.empno);
12. How to delete duplicate rows in a table?
    delete from emp a where rowid != (select max(rowid) from emp b where a.empno=b.empno);
13. Count of number of employees in department wise.
    select count(EMPNO), b.deptno, dname from emp a, dept b where a.deptno(+)=b.deptno group by b.deptno,dname;

14. Suppose there is annual salary information provided by emp table. How to fetch monthly salary of each and every employee?

    select ename,sal/12 as monthlysal from emp;

15. Select all record from emp table where deptno =10 or 40.

    select * from emp where deptno=30 or deptno=10;

16. Select all record from emp table where deptno=30 and sal>1500.

    select * from emp where deptno=30 and sal>1500;

17. Select all record from emp where job not in SALESMAN or CLERK.

    select * from emp where job not in ('SALESMAN','CLERK');

18. Select all record from emp where ename in 'BLAKE','SCOTT','KING'and'FORD'.

    select * from emp where ename in('JONES','BLAKE','SCOTT','KING','FORD');

19. Select all records where ename starts with ‘S’ and its lenth is 6 char.

    select * from emp where ename like'S____';

20. Select all records where ename may be any no of character but it should end with ‘R’.

    select * from emp where ename like'%R';

21. Count MGR and their salary in emp table.

    select count(MGR),count(sal) from emp;

22. In emp table add comm+sal as total sal .

    select ename,(sal+nvl(comm,0)) as totalsal from emp;

23. Select any salary <3000 from emp table.

    select * from emp where sal> any(select sal from emp where sal<3000);

24. Select all salary <3000 from emp table.

    select * from emp where sal> all(select sal from emp where sal<3000);

25. Select all the employee group by deptno and sal in descending order.

    select ename,deptno,sal from emp order by deptno,sal desc;

26. How can I create an empty table emp1 with same structure as emp?

    Create table emp1 as select * from emp where 1=2;

27. How to retrive record where sal between 1000 to 2000?
    Select * from emp where sal>=1000 And sal<2000

28. Select all records where dept no of both emp and dept table matches.
    select * from emp where exists(select * from dept where emp.deptno=dept.deptno)

29. If there are two tables emp1 and emp2, and both have common record. How can I fetch all the recods but common records only once?
    (Select * from emp) Union (Select * from emp1)

30. How to fetch only common records from two tables emp and emp1?
    (Select * from emp) Intersect (Select * from emp1)

31. How can I retrive all records of emp1 those should not present in emp2?
    (Select * from emp) Minus (Select * from emp1)

32. Count the totalsa deptno wise where more than 2 employees exist.
    SELECT deptno, sum(sal) As totalsal
    FROM emp
    GROUP BY deptno
    HAVING COUNT(empno) > 2