What are joins..how many types of joins are there?

A join is a query that extracts data from two or more tables, views or snapshots.

1. Equi Join
2.Non Equijoin
3.outer joins
4.self joins 

To get second highest age from the student table 

SELECT * FROM (SELECT s.*, rownum age_rank

 FROM (SELECT * FROM student ORDER BY age DESC) s) WHERE age_rank=2; 

Write a query to display alternate records from the employee table?

SELECT * FROM emp WHERE ROWID IN 

(SELECT DECODE(MOD(ROWNUM,2),0,ROWID, NULL) FROM emp);