2014 Soft Test Database System engineer Knowledge Point explanation (2)

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　　Structure of the database system

1. The three-level schema structure of the database system

The three-level mode structure of the database management system is composed of the outer mode, the mode and the inner mode. External mode (external schema), or sub-schema (Subschema) or user schema, refers to the data structure that the database user sees, and is the view of the data that the user sees. Schema (Schema) or logical schema (logic schema) is a description of the logical structure and characteristics of the entire data in the database, and is the sum of the views of the data that all users see. The outer mode is part of the pattern. The internal schema (internal schema), or the storage mode (storage schema), or the physical schema (physical schema), refers to the representation of the data on the storage medium within the database system, that is, the physical structure of the data and the way the access is described. The schema describes the global logical structure of the data, which determines the function of the DBMS. The outer pattern involves only a local logical structure, a subset of patterns, and a call to the schema. The three-level model of a database system is a tool for three levels of abstraction of data. With the three-level model, the specific organization of the data is left to the DBMS, allowing the user to process the logical structure of the data at a high level without having to care about the physical structure of the data. In order to achieve these three levels of this connection, the DBMS provides two images in the communication three-level mode: External mode-pattern-image Mode-Internal mode image

(1) Pattern mode specifies a logical representation or description of all data in a database. The pattern is different from the inner mode and the outside mode. It is more abstract than the internal schema and does not involve the physical structure of the data and storage media. It is not related to specific applications or high-level languages.

(2) out-of-mode mode is the data view of individual users, that is, the logical representation of data related to an application.

(3) Inner mode mode is the internal representation or the underlying description of all databases, and is used to define the physical structure and access mode of the data.

(4) level two images can have any number of external modes for the same pattern. External mode-The image of a pattern defines the correspondence between an outer pattern and a pattern. The definitions of these images are usually contained in their own outer patterns. When the pattern changes, the image of the external mode-mode is changed accordingly to guarantee the invariance of the outer mode. This is the responsibility of the database Administrator (DBA). The image of the pattern-inner mode defines the correspondence between the data logical structure and the storage structure. For example, the internal representation of a field, and so on. When the storage structure of the database changes, the image of the pattern-inner mode must be modified accordingly so that the pattern remains invariant. This is still the responsibility of the DBA. It is because of the above two-level image function and its automatic implementation, the data in the database system has high logical independence and physical independence, which greatly facilitates the user's use.

(5) conceptual model and model the ANSI Study Group presented an interim report in February 1975. A final report, called the Anˉsi/sparc Report, was presented in 1978, referred to as the SPARC report. The SPARC report indicates that the database management system should have a three-level pattern structure, namely, conceptual mode, external mode and internal mode. The outer model and the internal model are much the same as above. However, the conceptual model in SPARC reports refers to the real-world model of an organization or department, i.e. the conceptual model. The conceptual pattern describes only entities and their attributes and relationships, and does not involve the concept of the machine world. The conceptual model is the structure of information in the information world, and the model is the logical representation of the conceptual model in the machine World category. A conceptual model is independent of a specific computer system, or even a machine-independent, model of an organization or department. A common method of describing conceptual patterns is the E-r method. The pattern is dependent on the specific computer and its DBMS. Patterns are implemented through three specific models: hierarchical model, mesh model, and relational model. The conceptual pattern must be translated into a specific data model before it can be implemented on the appropriate DBMS. Both conceptual models and patterns describe the overall structure of information or data, but they are described at different levels of abstraction. The conceptual model is far from the machine and has a low degree of formality. From a point of view away from the machine, the conceptual model is more abstract, but closer to the real world. The pattern description uses a formalized language-schema DDL, whereas the conceptual model describes the use of e-r diagrams and some natural languages.

2. Using the database's computer system structure

Broadly speaking, the computer system that actually uses the database is made up of the following parts:

(1) The hardware part includes host, external storage device, network equipment and power supply, etc.

(2) The software includes operating system, DBMS, compiling system and application development tool software.

(3) The personnel includes the database administrator (data Base administrator-dba), the Systems Analyst (System Anaˉlyst), the application programmer (Application Programmer), and the Users (user).

3. Database management System

DBMS of database management system is the core of database system. DBMS is the software that is responsible for the establishment, use and maintenance of the database. The DBMS is built on the operating system and implements unified management and control of the database. The various database commands used by the user, as well as the execution of the application, must eventually pass through the DBMS. In addition, the DBMS undertakes the security protection of the database and guarantees the integrity and security of the database according to the requirements of the DBA. The main functions of the DBMS include the following key aspects:

(1) The database definition function DBMS is described by providing a data description language (also known as the data Definition language) (descripˉtive Language ——— DDL) for external patterns, patterns, and internal schemas. Then the pattern translation program uses the DDL to write the various patterns of the definition of the source code translated into the corresponding internal expression, the formation of the corresponding target form, called the target outside the model, target mode, target mode, these target patterns are the database description, not the data itself. The (target) pattern depicts only the form or framework of the database, not the contents of the database. These target patterns are stored in a data dictionary (or system target) as a basic basis for DBMS access and management data. For example, the DBMS defines a physical and logical structure based on these schema definitions, and maps the logical structure and user view to export the data to be retrieved by the user.

(2) Data manipulation The DBMS provides data manipulation language (manipulation Language ——— DML) to implement some basic operations on data in the database, such as: retrieval, insertion, modification, deletion, sorting, and so on. There are two types of DML: one is embedded in the main language, such as embedded in a high-level language such as C or COBOL. Such DML languages themselves cannot be used alone, so they are called host-type DML or embedded DML. The other is a non-embedded language (including interactive command language and structured language), its syntax is simple, can be used independently, by a separate interpretation or compilation system to execute, so it is generally called autonomous or self-contained DML. The command language is the line structure language, executed in a single order. Structured language is the expansion or development of command language, which increases the function of program structure description or process control, such as Loop and branch. The command language is generally explained by article-by-clause execution. A structured language can interpret execution, or it can be compiled and executed. Now the DBMS generally provides the command language interactive environment and the structure environment two modes of operation, for the user to choose. The DBMS controls and executes DML statements (or DML programs) to complete operations on the database. For autonomous structured Dml,dbms, the method of interpreting execution is usually used, but there are methods of compiling and executing, and more and more compiling is performed. In addition, many systems also have the interpretation and compilation of two functions, the user selected one. There are two methods for embedding or shrinking the main type of Dml,dbms:

① pre-compilation method.

② modifies and expands the main language compiler (also known as the Enhanced compilation method). The precompiled method is to provide a preprocessor by the DBMS that scans the source for syntax, identifies DML statements, and translates these statements into special invocation statements in the main language. The main language must have a calling interface with DML. This creates an executable target with the main language statement as the connection forms the target.

(3) The dynamic management of database running management database is the core part of DBMS, including concurrency control, access control (or security check, integrity constraint checking), maintenance inside database (such as index, automatic maintenance of data dictionary, etc.), buffer size setting and so on. All the database operation is under the unified management of this control section, work together to ensure the normal operation of transaction processing, ensure the correctness of the database, security and effectiveness.

(4) Establishment and maintenance of database the establishment and maintenance of database includes initial data loading, database dump or fallback function, database recovery function, database re-organization function and performance analysis functions, which are usually completed by the corresponding functional subroutine. The DBMS varies depending on the software product and version. Typically, the DBMS on a mainframe is the most functional, and the DBMS on a small machine has a slight weakness, and the DBMS on the microcomputer is weaker. However, at present, due to the improvement of hardware performance and price, the DBMS function of microcomputer is more and more complete.

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