You create a view from one or more base tables or from other views.
You can create hierarchies of views in which views are created from other views, but deleting lower-level views invalidates dependencies of higher-level views in the hierarchy.
A view usually presents only a subset of the columns and rows in the base table or tables.
Some view columns do not exist in the underlying base tables. For example, a view column can show a data summary (such as an average), which you cannot directly obtain from a base table.