51、在數據庫系統中，使數據恢復到故障發生前的一致狀態的機制稱為（  ）。

A、恢復機制

B、備份機制

C、封鎖機制

D、事務機制

52、通過將一個關系拆分成兩個更小的關系來使其滿足范式時，必須（  ）來保持數據的完整性約束。

A、用相同的屬性使兩個子關系互相關聯

B、移除兩個子關系中所有的函數依賴

C、封鎖機制

D、事務機制

53、下列描述中，（  ）不是分布式數據庫數據透明性的表現形式。

A、代碼透明性

B、分片透明性

C、位置透明性

D、模型透明性

54、分布式數據庫的 CAP理論指的是：對于一個分布式數據庫系統，一致性、可用性和分區容錯性這三個特點，最多只能滿足（  ）個。

A、0

B、1

C、2

D、3

55、NOSQL數據庫的四大分類是指（  ）。

A、鍵值存儲數據庫，列存儲數據庫，文檔型數據庫，關系型數據庫

B、列存儲數據庫，文檔型數據庫，關系型數據庫，分布式數據庫

C、鍵值存儲數據庫，列存儲數據庫，文檔型數據庫，圖數據庫

D、列存儲數據庫，文檔型數據庫，關系型數據庫，圖數據庫

56、A database system is a ollction of itererelated data and a set of programs that allow users to access and modify these data. A major purpose of a database system is to provide users with an（  ）view of the data. That is, the system hides certain details of how the data are stored and maintained. For the system to be usable, it must retrieve data（   ）。 The need for efficiency has led designers to use complex data（  ）to represent data in the database. Since many database-system users are not computer trained, developers hide the complexity from users through several levels of abstraction, to simplify users' interactions with the system. Physical level is the lowest level of abstraction that describes（  ）the data are actually stored. Logical level is the next-higher level of abstraction that describes（  ）data are stored in the database, and what relationships exist among those data. View level is the highest level of abstraction that describes only part of the entire database.

A、 abstract

B、 physical

C、 administrator

D、 operator

A、 completely

B、 safely

C、 usefully

D、 fficiently

A、 files

B、 structures

C、 graphs

D、 flows

A、 how

B、 what

C、 which

D、 when

A、 how

B、 what

C、 which

D、 when