Introduction to "Introduction to Database System" Introduction to----Database System (III.)

1.2.7 Relationship Model

The relational model is one of the most important data models at present.

I. Data structure of relational model

A relational model consists of a set of relationships, and the data structure of each relationship is a normalized two-dimensional table.

Relationship: A relationship usually corresponds to a table
Tuples: A single row in a table is a tuple
Properties: one column
Code: An attribute group can uniquely determine a tuple
Domain: The value range of a property
Component: A property value in a tuple
Relational schema: Described by the table model: the relationship name (attribute 1, property 2, ...). Attribute N)

The relationship model requirements must be normative, that is, the requirement relationship satisfies certain normative requirements (paradigm requirements).

The manipulation and integrity constraints of the relational data model

The main operations are: inserting, deleting, querying and updating data

Integrity constraints include: ① entity integrity ② Referential integrity ③ user-defined integrity

The relational model hides the storage path, and the user simply points out "what" or "what to look for" without needing to elaborate.

How to do so greatly increases the independence of the data and improves the productivity of the user.

Three, the storage structure of relational data model

Some DBMS one table corresponds to a system file, and some DBMS designs the table in several large files, builds the index

Iv. advantages and disadvantages of relational data models

Pros: ① The relationship model is based on a rigorous mathematical concept model, unlike the format model.

Seniors single of ② relationship model

The ③ storage path is transparent, resulting in higher data independence, better security confidentiality, and simplified programmer work and

According to the work of the library development

Disadvantage: Query efficiency is not as good as formatted data model, DBMS design is very difficult

1.3 Database system Structure 1.3.1 The concept of database system pattern

The concept of type and value: type refers to the description of the structure and properties of a class of data, and the value is a specific assignment.

Pattern: is a description of the logical structure and characteristics of all data in the database, not only the description of the type, but also the specific value.

A specific value of the pattern is called an instance of the pattern.

Three-level schema structure of 1.3.2 database system

The three-level schema structure of the database refers to the database system is composed of the external mode, the mode and the internal mode three level

First, mode (logical mode)

is a characteristic description of the logical structure of all data in a database, and is a public view of all users

A database has only one logical mode

Second, outer mode (sub-mode & user mode)

is a description of the local data logic and characteristics that the database user (including the programmer and the end user) can see and use,

is a data view of a database user, which is a logical representation of the data associated with an application.

Three, internal mode (storage mode)

Is the description of the physical structure and storage of the data, which is the representation of the data inside the database.

Two-level image function and data independence of 1.3.3 Database

The database management system provides a two-layer image between these three-level modes:

External mode/mode image
Mode/Internal mode image

It is these two layers of images that ensure that the data in the database system can have high logical independence and physical independence.

One, outer mode/mode image

The pattern describes the global logical structure of the data, and the external mode is the local logical structure of the data.

Second, mode/internal mode image

When the storage structure of the database has changed, the database administrator changes the mode/internal mode image Accordingly, so that the pattern can be kept unchanged.

Thus, the application does not have to make changes to ensure the physical independence of data and data, referred to as the physical independence of data.

In a three-level schema structure, the database schema, the global logical structure, is the center and key of the database, independent of the other layers of the database. Set

The first task of the accounting database is to determine the logical structure of the database.

1.4 The composition of the database system one, the hardware platform and the database

Requirements: The memory is large enough, large enough external memory equipment, good channel sex

Second, the software

(1) DBMS. DBMS is the system software for establishing, using and maintaining the database.

(2) operating system that supports DBMS operation

(3) have a high-level language with the Database interface and its compilation system, easy to develop applications.

(4) DBMS-centric application development tools

(5) database application system developed for a specific application environment

Third, personnel

1. Database administrator

Determine the content and structure of information in a database

Determine the storage structure and access policies in the database

Defining security requirements and integrity constraints for data

Monitoring the use and operation of the database

The improvement of database and reconstruction of reorganization

2. System Analyst and Database Designer

The system analyst is responsible for the application requirements analysis and specification, to be combined with the user and DBA,

Determine the software configuration of the system and participate in the summary design of the database system.

Database Designer is responsible for the data in the database determination, the design of the database at all levels, generally by

DBA as.

3. Application Programmer

4. User

User access to the database interface is usually: Browser, menu driver, table operation, graphical display, report writing and so on.

(1) Accidental user

(2) Simple user

(3) Complex users

To the end of the first one here, the main is to briefly introduce what is the database and the type of database

Starting from the next chapter is the key to this book. Study of relational database

Introduction to "Introduction to Database System" Introduction to----Database System (III.)