Views and corelated query

Question

Hi ,
1> When are views useful?(scenarios where u could use)
2> when are corelated subqueries used?(scenarios where u could use)
Thanks in advance

Answer 1

When are views useful?
A view is simply the representation of a SQL statement that is stored in memory so that it can easily be re-used. For example, if we frequently issue the following query
SELECT empid FROM emp;
I might well want to make this a view (the reality is that we would probably never create a view for a statement this simple but we wanted to use an easy example).
To create a view use the create view command as seen in this example
CREATE VIEW view_emp
AS
SELECT empid FROM emp;
This command creates a new view called VIEW_EMP
Benefits of Oracle Views
* Commonality of code being used.
* Security.
* Predicate pushing
when are corelated subqueries used?(scenarios where u could use)
A correlated subquery is a subquery that uses values from the outer query. The Oracle database wants to execute the subquery once and use the results for all the evaluations in the outer query. With a correlated subquery, the database must run the subquery for each evaluation because it is based on the outer query’s data.
SQL> select
2 book_key,
3 store_key,
4 quantity
5 from
6 sales s
7 where
8 quantity < (select max(quantity)
9 from sales
10 where book_key = s.book_key);
BOOK_K STOR QUANTITY
B101 S101 1000
B102 S102 10
B102 S103 200
B116 S105 100
B101 S105 8000
B109 S109 100
81 rows selected.
In the example above, the subquery references the book_key in the outer query. The value of the book_key changes by row of the outer query, so the database must rerun the subquery for each row comparison. This has a significant performance impact on the execution time of the query, and for that reason, correlated subqueries should be avoided if possible.
The outer query knows nothing about the inner query except its results. For that reason, the outer query cannot reference any columns in the subquery. However, the subquery has access to the outer query and can reference outer query columns, thus the correlated subquery.