Complex Queries in SQL ( Oracle )

--> 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