1.A primary key is a field (or group of fields) that uniquely describes each record in the database. T
2.Data redundancy improves the integrity of a database. 
3.SQL is the language used by relational databases to create objects and to manipulate and retrieve data.
4.A relational database management system does not include tools for backing up & restoring databases.
5.1，一种主键是一个字段（或字段组）唯一地描述了每个记录在数据库中。Ť
6.2.数据冗余改善了数据库的完整性。
7.3，SQL是关系型数据库用于创建对象，并以操作和检索数据的语言。
8.4.关系数据库管理系统并不包括用于备份和恢复数据库的工具。
9.An attribute is also known as a row in most databases.
10.An association between entities is known as a relationship.
11.Integrity constraints limit the number of entities that can be placed in a table or database.
12.The Entity-Relationship data model is often used in the physical design phase.
13.The concept “relation” in relation model is exactly the same as the concept “relationship” in ER model.
14.9.一种属性也被称为一个行中大多数数据库。
15.10.实体之间的关联称为关系。
16.11.完整性约束限制可以被放置在一个表或数据库实体的数目。
17.12.实体关系数据模型中经常使用的物理设计阶段。
18.13.在关系模型的概念'关系'是完全一样的ER模型的概念的“关系”。
19.Most relationship sets in a database system involves two entity sets.
20.The closure of an attribute set contains that attribute set.
21.Lossless decomposition is necessary in a decomposition algorithm.
22.If A → B and C → D hold, then AC → BD also holds.
23.It is not necessary that a legal schedule preserves the order in which the instructions appear in each individual transaction.
24.Update operations in database must be written into log before updating database.
25.19.大多数关系套在数据库系统中包括两个实体集。
26.设置属性20.关闭包含的属性设置。
27.21.无损分解是必要的分解算法。
28.22.如果A→B和C→d握住，然后AC→BD还持有。
29.23.这是没有必要，一个法律时间表保留在其中的指令出现在每个单独的事务处理的次序。
30.在数据库24的更新操作必须写入日志更新数据库之前。
未知答案：
1.At its simplest, a database can be seen as a collection of related data.
2.If a salesperson attempts to order merchandise for a customer not in the customer service database, the database will typically generate an error message. This message indicates that an integrity constraint has been violated.
3.数据库管理员英文An advantage of using a relational database is to facilitate consistency.
4.With the six basic operations (union, difference, selection, projection, product, and renaming), relational algebra is complete. Other operations are just syntactic sugar and can be derived from the basic operations.
5.Once a transition is committed, the changes it has made to database persist across system crashes, that is, consistency.
6.1.在其最简单的，数据库可以被看作是相关的数据的集合。
7.2.如果营业员试图订购商品的顾客不是在客户服务数据库，该数据库将典型地产生一个错误信息。此消息表明完整性约束已经被破坏。
8.3.使用关系数据库的一个优点是便于一致性。
9.4.在六种基本操作（集，差，选择，投影，产品和重命名），关系代数完成。其他的操作都只是语法糖，并且可以从基本操作得到的。
10.5.一旦过渡被提交，已作出的数据库中的改变持续整个系统崩溃，即，一致性。
1. _______ is the most widely used data model
A. relational model          B. entity-relationship model
C. object-based data model    D. semi-structured data model
是最广泛使用的数据模型
A.关系模型B.实体关系模型
C.基于对象的模型数据D.半结构化的数据模型
一个关系模式是一组_______。
A.元组B.属性C.列D.行
2. A relation schema is a set of _______.
A. tuples  B. attributes  C. columns    D. rows
3.一组属性，这些属性，采取集体，使我们能够唯一标识关系中的一个元组被称为______的。
A.超键B.主键C.候选键D.外键
3. A set of attributes that, taken collectively, allow us to identify uniquely a tuple in th
e relation is called ______.
A. super key B. primary key  C. candidate key  D. foreign key
4.应用程序用户在_______级数据交互。
A.物理B.逻辑C.查看D.以上所有的
4. Application users interact with data in the _______ level.
A. physical    B. logical  C. view  D. all of the above
5.关系代数是_____语言
A.程序B.声明C.自然D.商业
5. Relational algebra is a _____ language
A. procedural   B. declarative  C. natural  D. commercial
6.关系代数的基本操作包括________。
A.广义投影，聚合函数，外部联接
B.设置交集，自然连接，分立，转让
C.选择，项目设置工会，设置不同，笛卡尔积，重命名
D.设置路口，设置工会，设置不同的
6. The fundamental operations of relational algebra includes_______.
A. generalized projection, aggregate function, outer join
B. set-intersection, natural join, division, assignment
C. select, project, set-union, set-difference, Cartesian product, rename
D. set-intersection, set-union, set-difference
7.如果表S有M列，表T具有N列，那么S和T的笛卡尔积具有____列。
A.M。* N B. M-N C. M + N D. M + N-1
7. If table S has M columns, table T has N columns, then the Cartesian product of S and T has ____ columns.
A. M*N    B. M-N  C. M+N    D. M+N-1
8.关系代数的______操作需要两个参与关系有属性的数相同。
A.师B.参加C.笛卡尔积D.单位
8. The ______ operation in relational algebra requires the two participating relations have the same number of attributes.
A. division  B. join  C. Cartesian product  D. union
9. _____不包含在SQL语言？
A.查询评估B.完整性C​​.查看D.授权
9.  ______is not contained in SQL language?
A. query evaluation  B. integrity C. view  D. authorization

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