What is the logical structure of database file

What is the logical structure form of database file

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The logical structure of the database file is a record file . The logical structure of documents can be divided into two categories ：1、 Unstructured streaming files , It means that the information in the document is no longer divided into units , It is a file composed of a string of character streams in turn ;2、 Structured record files , It is the user who divides the information in the document into information units according to logically independent meanings , Each unit is called a logical record . The files studied by the database are a collection of records with structure , Each record can consist of several data items , Therefore, the logical structure of the database file is a structured record file .

The operating environment of this tutorial ：windows7 System 、Dell G3 The computer .

The logical structure of the file is the user visible structure .

Logical files are structured in two forms ：

• One is unstructured streaming files , It means that the information in the document is no longer divided into units , It is a file composed of a string of character streams in turn .

• One is a structured record file , It is the user who divides the information in the document into information units according to logically independent meanings , Each unit is called a logical record （ Abbreviation record ）.

All records usually describe a set of entities , Having the same or different number of data items , The length of records can be divided into fixed length and variable length records .

The logical structure of the database file is a structured record file .

Structured record files

When the operating system or database is running , The computer will record some information in the file , These files can be called record files . The record is the basic unit of access in the file .

file (File) Is a collection of records of the same nature , The data volume of files is usually large , Placed on external memory . It can be divided into two categories according to the types of records ： Operating system files and database files .

One of the files studied by the operating system is the unstructured streaming file , It means that the information in the document is no longer divided into units , It is a file composed of a string of character streams .

The file studied by the database is a collection of records with structure , Each record can consist of several data items . Or according to the length characteristics of the record , Documents are divided into fixed length record documents and variable length record documents . Each record in the file contains the same length of information , It is called fixed length record file , Otherwise, it is called indefinite length record file

Record ：

A record is a collection of related data items , Used to describe the properties of an object in some way . What data items should a record contain , It depends on which aspect of the object needs to be described . And an object , Because of his different environment, he can be regarded as a different object . for example , One of the students , When taking him as a student in the class , The student number should be used in his description 、 full name 、 Age and department class , It may also include the name of the course he has studied 、 Results and other data items . But if students are treated as a medical object , For the data items described by him, such as medical record number 、 full name 、 Gender 、 date of birth 、 height 、 weight 、 Blood pressure and medical history .

In many records , In order to uniquely identify a record , Must be in each data item of a record , Identify one or more data items , Call their set keywords (key). Or say , Keywords are the only data items that can identify a record . Usually , Just use a data item as a keyword . for example , The previous medical record number or student number can be used to identify a unique record from many records . However, sometimes such data items cannot be found , We have to define several data items as keywords that can uniquely identify a record among many records .

The logical and physical structure of records

• The logical structure of records refers to the way they are presented to users or applications , It is the user's representation and access of data .

• The physical structure of a record is the way data is stored in physical memory , Is the physical representation and organization of data .

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