If a database administrator omits the CONSTRAINT clause, Oracle will automatically assign the constraint a system-generated name that will not be very meaningful. It is a simple SELECT query with no functions or group clause,but just selection of columns from the table without any transformation.
A database also stores the execution plan for creating a view-this means that data can be retrieved rapidly through use of a view even though the actual data presented by a SELECT query of a view is not stored as part of a view.
Rather,the data is "gathered together" each time that a view is queried from the database tables for which a view is defined-these are termed base tables. From the syntax, The FORCE option allows a view to be created even if a base table that the view references does not already exist.
Sequences are special database objects that support the automatic generation of integer values,and are often used to generate primary key values for tables.
Indexes are created on table columns to facilitate the rapid retrieval of information from tables.
A Complex view is created on multiple tables using joins. But since the view is on multiple data and selection of columns is also not simple, it does not allow DML operation on it.