Four basic database concepts
data
- Defining data: records of a transaction descriptor
data base stored in the object database - The kind of data ( not only refers to digital ): numeric, text, graphics, audio, video, student records and other archives
- Meaning of the data referred to semantic data , the data to the semantics are inseparable
Database (DB)
- The definition of the database: The database is long-term storage in the computer , organized , can share the large amounts of data collection.
Database management system (DBMS)
- Definition database management system:
(1) a layer positioned between the user operating system and the data management software
(2) is the basic software, it is a large and complex software system - Commonly used database management system
SQL Server, Oracle, My SQL - The main functions of database management system
(1) data definition function
(2) data organization, storage, management, and
(3) data manipulation functions
(4) transaction management and operation management database
(5) to establish and maintain a database function
Database System (DBS)
- Database system including: database, database management systems (and application development tools), applications, databases administrator
The basic concept of information in the world
entity
The objective existence of things can be distinguished from each other entities called.
Entity type
Entity name and attribute name and describe the same set of abstract entity called entity type.
Entity set
Set of the same type of entity is called an entity set.
contact
Between the internal relations of things and things in the real world reflected the link between internal and entities associated with entities in the information world.
Relational Model of Data Structure
relationship
A corresponding relationship often said that a table.
Tuple
Row in the table that is a tuple.
Attributes
One that is a property of the table, since each attribute to a name that is the attribute name
code
Also known as code keys. Table a set of attributes that can uniquely identify a tuple.
area
Is a group having the same set of data worth type. The range of attributes from a domain.
Components
Attribute value of a tuple.
Relational schema
Description of the relationship.
Three mode configuration database system
- Structure of the database has three modes : mode, external mode, the mode
- Advantages : three model database systems is three levels of abstraction of data, the organization-specific data that the DBMS left, allows the user to logically, an abstract process data, without concern for the specific data representation in the computer and storage. In order to achieve these three levels of abstraction contacts and internal conversion, the database management system provides a mapping between the two layers of three modes: an outer mode / image mode, mode / the image mode. Mode / image mode to ensure the physical independence of the data and procedures. External mode / image mode to ensure the independence of the logic of data and programs.
- Defined patterns:
(1) the logical structure of data (data item name, the type, range, etc.)
(2) between the data link
(3) data security, integrity requirements relevant - Defines an outer model (also called sub-mode or user mode):
(1) describe the logical structure and characteristics of the local data used by the user
(2) views the user database, and an application is a logical representation of data relating to - Define the mode (also called storage mode):
(1) a description of the physical structure and the data storage
(2) is a representation of data within the database - Data Definition Language
is focus on language SQL language for data definition and database object structure defined by CREATE, ALTER and DROP syntax composed of three, was first started by Codasyl (Conference on Data Systems Languages) data model, it is now incorporated into the SQL command as a subset. Most DBMS support DDL operations on database objects, part of the database (eg PostgreSQL) can be placed in the DDL transaction instruction, that is, it can be withdrawn (Rollback). Newer versions of the DBMS will join the DDL trigger special program that allows database administrators to track changes from the DDL. - Data Manipulation Language
data manipulation language, the user can achieve the basic operation of the database through it. - Physical independence of the data and programs
(1) when the storage structure of the database changes (e.g., another storage structure selected), the database administrator edit mode / image mode within the mode remains unchanged.
(2) applications are not affected. Ensure the independence of physical data and procedures, referred to as physical data independence. - Logical data independence and procedures
(1) When the mode is changed, the external database administrator mode / image mode to be changed accordingly, so that the outer mode remains unchanged.
(2) the application is based on data prepared by the external mode, without having to modify the application to ensure the independence of the data and logic program, referred to as the logical data independence. - Why Database independence of data and programs
because of the database, the application access to the data is to be carried out by the DBMS, you can not directly manipulate the data itself. And DBMS mechanism provides two images: an outer mode / image mode and a mode / the mode map. This mechanism ensures that the image layers logic independence and physical independence of data in the system database.
