CH2 - Database system concepts and architecture

Question 1

Data model

Answer 1

a set of concepts to describe the structure of a database, the operations for manipulating these structures, and certain constraints that the database should obey

Question 2

 Data model structure and constraints

Answer 2

constructs are used to define the database structure. They typically include elements (and their data types) as well as groups of elements (entity, record, table) and relationships among such groups. Constraints specify some restrictions on valid data ; these constraints must be enforced at all times.

Question 3

 Data model operations

Answer 3

used for specifying database retrievals and updates by referring to the constructs of the data model. Operations on the data model may include basic model operations (generic insert, delete, update) and user-defined operations

Question 4

 Conceptual (high-level, semantic) data models

Answer 4

provide concepts that are close to the way many users perceive data. It is also called entity-based or object-based data models

Question 5

 Physical (low-level, internal) data models

Answer 5

provide concepts that describe details of how data is stored in the computer. These are usually specified in an ad-hoc manner through DBMS design and administration manuals.

Question 6

 Implementation (representational) data models

Answer 6

provide concepts that fall between the above two, used by many commercial DBMS implementations.

Question 7

 Self-describing data models

Answer 7

combine the description of data with the data values (XML, key-value stores and some NOSQL systems).

Question 8

 Database schema

Answer 8

the description of a database which includes descriptions of the database structure, data types and the constraints on the database

Question 9

 Schema diagram

Answer 9

an illustrative display of (most aspects of) a database schema

Question 10

 Schema construct

Answer 10

a component of the schema or an object within the schema

Question 11

 Database state

Answer 11

the actual data stored in a database at a particular moment in time. This includes the collection of all the data in the database. It is also called the database instance, occurrence or snapshot. It should be noted that the term instance is also applied to individual database components. Database state refers to the content of a database at a moment in time.

Question 12

 Initial database state

Answer 12

refers to the database state when it is initially loaded into the system

Question 13

 Valid state

Answer 13

state that satisfies the structure and constraints of the database.

Question 14

 Three-schema architecture

Answer 14

proposed to support DBMS characteristics of program-data independence and support of multiple views of the data. It is not explicitly used in commercial DBMS products, but has been useful in explaining database system organization. The three-schema architecture defines DBMS schemas at three levels : internal, conceptual and external schemas

Question 15

• Internal schema

Answer 15

at the internal level to describe physical storage structures and access paths (typically uses a physical data model)

Question 16

• Conceptual schema

Answer 16

at the conceptual level to describe the structure and constraints for the whole database for a community of users (uses a conceptual or an implementation data model)

Question 17

• External schema

Answer 17

at the external level to describe the various user views (usually uses the same data model as the conceptual schema)

Question 18

 Logical data independence

Answer 18

the capacity to change the conceptual schema without having to change the external schemas and their associated application programs

Question 19

 Physical data independence

Answer 19

the capacity to change the internal schema without having to change the conceptual schema.

Question 20

 Data Definition Language (DDL)

Answer 20

used by the DBA and database designers to specify the conceptual schema of a database. In many DBMSs, the DDL is also used to define internal and external schemas (views). In some DBMSs, separate storage definition language (SDL) and view definition language (VDL) are used to define internal and external schemas.

Question 21

 Data Manipulation Language (DML)

Answer 21

used to specify database retrievals and updates. DML commands (data sublanguage) can be embedded in a general-purpose programming language (host language) such as COBOL, C, C++ or Java. A library of functions can also be provided to access the DBMS from a programming language. Alternatively, stand-alone DML commands can be applied directly (query language)