Database system components

Database System Overview

Database system (DBS) is a database, the database management system (and its application development tools), application and database administrator (DBA) consisting of storage, management, and maintenance of data processing systems
Database Management System (the DBMS) : Core Software
- Features: data definitions; data organization, storage and management; data manipulation; transaction management and operational management of the database; the establishment and maintenance of the database; others
Database (DataBase): DB
- Is a long-term storage in a computer, an organized collection of large amounts of data can be shared
Database Users
- The first category of users: the end-user (End User)
- The second class of users: Application Programmer (Application Programmer)
- The third category of users: database administrator (DataBase Administrator): DBA

In the database using this tool to model data abstraction, representation and processing of data and information in the real world
Category: conceptual model : for DBMS to achieve network model, hierarchical model, relational model, object-oriented model, object-relational model

Logical and physical models : the bottom of the abstract data described in the data representation and access within the system
Constituent elements: a data structure; data operation; integrity constraints;

Contact within a single entity type

Certain employees (cadres) "lead" a number of employees, a worker is only the direct leadership of another worker

Entity type: rectangle, rectangle write the name of the entity
Properties: oval, with no connection to the edge
Contact: diamond, box indicate the contact name, with no connection to the edge, and mark the contact type (1: 1,1: n or m: n)

Integrity constraints:
- No values corresponding to the parent node can not be inserted child node value
- If the value deleted the parent node, the child node corresponding values are also deleted
- Update operations, should update all appropriate records to ensure data consistency
Storage structure
- Neighbor-joining: in a hierarchical tree preorder traversal order of sequentially recorded values of all adjacent memory
- Links Law
  - Children - Brother Links Law: Each record set two types of pointers, pointing to the far left and the children of brothers recently
  - Hierarchy sequence linker method: the tree in pre-order traversal sequence for each record link value
Advantages: simple and clear data structure; high query efficiency, performance is better than the relational model, is not less than the mesh model; support provides good integrity;
Cons: many to many unnatural links; multiple pairs of insert and delete restrictions, writing complex applications; Queries child node must pass through parent node; structure tends to close the command level programming;

No parent allows multiple nodes; a node can have multiple parents;
Hierarchical model is actually a special case of a mesh model
Integrity constraints
- Code: collection of data items that uniquely identifies the record;
- Contact is a link between the many parents with their children recorded record;
- Support some constraints between parents and children recorded record
To name for each contact, and pointed out that parents and children records associated with the contact record
Advantages: more directly describe the real world; good performance, high access efficiency;
Disadvantages: more complex structure, and with the expansion of application environments, more complex database structure is not conducive to end-users to master; DDL, DML language complex, which are not easy to use;

User point of view, the logical structure data in the relational model is a two-dimensional tables of rows and columns, transparent to the user access path
Contact between the entities and entities with a table to represent the table stored as files
Relationship: a table; tuple: line; properties: a;

Main Code: an attribute group can uniquely identify a tuple;

Domain: attribute range;

Component: a tuple attribute value;

Relational schema: one to one, one to many, many to many;
As well as the relationship does not allow table table~~(Prohibition of nesting dolls)~~
Integrity constraints: entity integrity; referential integrity; user-defined integrity;
Pros: based on strict mathematical basis of the concept; single concept; the access path of the relational model is transparent to users;
Disadvantages: Queries less efficient than non-relational data model; DBMS increase the difficulty of development;

Type: data structure and attribute of a class of instructions;

Found: is a particular type of assignment;
Mode: data management, a type described

Example: data
From the database management system to see: three model structure
From the database end users see: single-user configuration; master-slave configuration; distributed structure; client / server; browser / application server / database server multilayer structure

A database is only a pattern within a pattern and can have multiple modes outer
Mode (Logical Mode): description of all the data in a database logical structure and characteristics ; intermediate layer structure database system mode;
External mode (user mode or sub-mode): strong measures to ensure the security of a database; each user can only see and access the external data corresponding to the pattern;
The mode (storage mode): description of the physical structure and data storage; representing data within the database;

Two image patterns to achieve three links and internal conversion DBMS
External mode / image mode: logic to ensure data independence
- Each outer mold has a corresponding relation between the outer mode / image mode, the external mode and the mode is defined
- Defined in the respective image typically contains described external mode in
Mode / the mode map: guarantee the physical data independence
- It defines the correspondence relationship between the logical structure and the global data storage structure
- The image is usually included in the definition of mode described in