Requires MySQL 5 Support for views was added to MySQL 5. As such, this chapter is applicable to MySQL 5 or later only.
The best way to understand views is to look at an example. Back in Chapter 15, "Joining Tables," you used the following SELECT statement to retrieve data from three tables:
SELECT cust_name, cust_contact FROM customers, orders, orderitems WHERE customers.cust_id = orders.cust_id AND orderitems.order_num = orders.order_num AND prod_id = 'TNT2';
That query was used to retrieve the customers who had ordered a specific product. Anyone needing this data would have to understand the table structure, as well as how to create the query and join the tables. To retrieve the same data for another product (or for multiple products), the last WHERE clause would have to be modified.
Now imagine that you could wrap that entire query in a virtual table called productcustomers. You could then simply do the following to retrieve the same data:
SELECT cust_name, cust_contact FROM productcustomers WHERE prod_id = 'TNT2';
This is where views come into play. productcustomers is a view, and as a view, it does not contain any actual columns or data as a table would. Instead, it contains a SQL querythe same query used previously to join the tables properly.
Why Use Views
For the most part, after views are created, they can be used in the same way as tables. You can perform SELECT operations, filter and sort data, join views to other views or tables, and possibly even add and update data. (There are some restrictions on this last item. More on that in a moment.)
The important thing to remember is views are just that, views into data stored elsewhere. Views contain no data themselves, so the data they return is retrieved from other tables. When data is added or changed in those tables, the views will return that changed data.
Performance Issues Because views contain no data, any retrieval needed to execute a query must be processed every time the view is used. If you create complex views with multiple joins and filters, or if you nest views, you may find that performance is dramatically degraded. Be sure you test execution before deploying applications that use views extensively.
View Rules and Restrictions