Difference between Simple and Complex View in SQL

Prerequisite – SQL | Views
A View in SQL as a logical subset of data from one or more tables. Views are used to restrict data access. A View contains no data of its own but its like window through which data from tables can be viewed or changed. The table on which a View is based are called BASE Tables.

There are 2 types of Views in SQL: Simple View and Complex View. Simple views can only contain a single base table. Complex views can be constructed on more than one base table. In particular, complex views can contain: join conditions, a group by clause, a order by clause.

The key differences between these types of Views are:

Simple View	Complex View
Contains only one single base table or is created from only one table.	Conatins more than one base tables or is created from more than one tables.
We cannot use group functions like MAX(), COUNT(), etc.	We can use group functions.
Does not contain groups of data.	It can conatin groups of data.
DML operations could be performed through a simple view.	DML operations could not always be performed through a complex view.
INSERT, DELETE and UPDATE are directly possible on a simle view.	We cannot apply INSERT, DELETE and UPDATE on complex view directly.
Simple view does not contain group by, distinct, pseudocolumn like rownum, columns defiend by expressions.	It can contain group by, distinct, pseudocolumn like rownum, columns defiend by expressions.
Does not include NOT NULL columns from base tables.	NOT NULL columns that are not selected by simple view can be included in complex view.

Reference –
View – orafaq