What is the result of this query,
SELECT * FROM Employee E, Department D WHERE E.DEPTNO=D.DEPTNO

Answer:

In this query, there are two tables Employee and Department and E is the alias name for Employee table and D is the alias name of D. The Employee table contains Employee details and the Department table contains Departments and Employee Id's.
When you need to know in which department is the employee present then you can use this query.