Complex SQL Queries In Oracle

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);

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);

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);

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);

Select FIRST n records from a table.
select * from emp where rownum <= &n;

Select LAST n records from a table
select * from emp minus select * from emp where rownum <= (select count(*) - &n from emp);

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;

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;

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);

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);

Select DISTINCT RECORDS from emp table.
select * from emp a where  rowid = (select max(rowid) from emp b where  a.empno=b.empno);

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);

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;

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;

Select all record from emp table where deptno =10 or 40.
select * from emp where deptno=30 or deptno=10;

Select all record from emp table where deptno=30 and sal>1500.
select * from emp where deptno=30 and sal>1500;

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