When defining and using a view, remember that a view is just an SQL statement and is replaced by its definition when the query is run.

These are some common uses of views.

• They allow you to have a recurring SQL query or expression in one place for easy reuse.
• They can be used as an interface to abstract from the actual table definitions, so that you can reorganize the tables without having to modify the interface.

If a subquery is associated with a single query, consider using the WITH clause of the SELECT command instead of creating a seldom-used view.

In general, these uses do not require nesting views, that is, defining views based on other views.

These are two patterns of creating views that tend to be problematic because the view’s SQL is used during query execution.

• Defining many layers of views so that your final queries look deceptively simple.

  Problems arise when you try to enhance or troubleshoot queries that use the views, for example by examining the execution plan. The query’s execution plan tends to be complicated and it is difficult to understand and how to improve it.

• Defining a denormalized “world” view. A view that joins a large number of database tables that is used for a wide variety of queries.

  Performance issues can occur for some queries that use the view for some WHERE conditions while other WHERE conditions work well.