42. Which three statements are true regarding views? (Choose three.)
A. Views can be created only from tables.
B. Views can be created from tables or other views.
C. Only simple views can use indexes existing on the underlying tables.
D. Both simple and complex views can use indexes existing on the underlying tables.
E. Complex views can be created only on multiple tables that exist in the same schema.
F. Complex views can be created on multiple tables that exist in the same or different schemas.
Answer: BDF
A错误，视图是一种基于表或其它视图的逻辑表。
B正确，视图是一种基于表或其它视图的逻辑表。
C错误，简单视图和复杂视图都能使用相关表的索引。
D正确，同C.
E错误，可以是不同schema间。
F正确，复杂视图的子查询可以是相同或者不同的schema间的多张表。