2014 Soft Test Database System engineer Knowledge Point explanation (1)

51cto College, in the soft exam preparation season specially organized the "2014 Soft test-database Systems Engineering-Knowledge point explain" summary ", to help you pass the school smoothly! More software proficiency test Counseling and questions, please follow 51cto College-soft Test classification bar

View Totals:2014 Soft Test-Database Systems Engineering Division-Knowledge Point explanation "summary" 650) this.width=650; "alt=" Hot article "src=" http://www.exam8.com/images/ Hot.gif "style=" padding:0px;margin:0px;vertical-align:middle;border:0px; "/>

　　Data model

　1. Three elements of the data model

(1) A data structure data structure is a collection of object types (objects type) being studied. These objects and object types are the constituent components of a database. Generally can be divided into two categories: one is related to data types, content and other properties of the object, one is related to the relationship between the data of the object. The former such as data items and records in a mesh model, fields, attributes, and relationships in a relational model. The latter such as the relational model in a mesh model (set type). In the domain of database, the data model is usually named according to the type of structure, and then the database management system is classified. The data models, such as hierarchical structure, network structure and relational structure, are respectively called as hierarchical model, mesh model and relational model. Accordingly, the database is called as the hierarchical database, the mesh database and the relational database respectively.

(2) Data manipulation data operations are collections of operations that are allowed to be performed on instances (or values) of various object types, including operations and related operational rules. In the database, the main operations are search and update (including INSERT, delete, modify) two categories. The data model defines the definition and syntax of these operations (that is, the language in which these actions are used). The data structure is a description of the static characteristics of the system, and the operation is a description of the dynamic characteristics of the system. There are both connections and differences.

(3) Constraints on data constraints are the set of integrity rules. Integrity rules are the constraints and dependencies of data and their relationships in a given data model to limit the state and state of the database that conforms to the data model, and to ensure the correctness, validity, and consistency of the data.

　　2. Conceptual model

Data model is the core and foundation of database system. Each DBMS software is based on some kind of data model. In order to express the connection between the concrete things in the real world and the data model supported by DBMS, people must first abstract the real world and the relation between them into the conceptual model of the information world, then transform the conceptual model of the information world into the data model of the machine world. In other words, the objective object in the real world is abstracted into a kind of information structure first. This information structure does not depend on specific computer systems and DBMS. Then, the conceptual model is converted to a data model supported by a DBMS on a computer system. Therefore, the conceptual model is an intermediate level from the real world to the machine world. The real world is reflected in the human brain, and then people begin to recognize these things, and after the abstraction of choice, naming, classification and organization, they form a conceptual model and enter into the information world.

Users are concerned with the nature of things and things in the real world and their interrelationships. For example, a user might care about his customers and their attributes, such as customer addresses, bank accounts, and so on. Users are also concerned about their own orders, such as who ordered the goods, what to order and how much, and so on.

The system analyst (analyst) is also concerned about the real world, but the system analyst needs to analyze the user's information needs. As a result of demand analysis, the analyst must have a structured description of the requirements in the form of a document, which is an information model.

Entity entities are the basic elements that make up a database. An entity is a thing that exists to distinguish between the attributes of this thing and the connection of this thing to other things. An entity can be a person or a thing; it can be a real object, or it can be a concept; it can be the thing itself, or it can refer to the connection between things.

Attribute (Attribute) an entity can be characterized by several attributes. property is relative to the entity and is the attribute that the entity has.

The keyword (key) can uniquely identify an entity's collection of attributes called Keywords (or codes).

The range of domain properties is called a domain.

A set of common characteristics or attributes that an entity of entity type has is called a solid type. In general, entity names and their attributes are used to describe the entity type of a class of entities abstractly.

The collection of entity sets (entities set) of the same entity is called an entity set. For example, a student is an entity set. The name of the entity set is the entity type. For students and (school number, name, age, department, grade) are entity type, and students are the entity type (school number, name, age, department, grade) the name of the two refers to the same objective object. But undergraduates and postgraduates can be of the same entity type, while the entity set is different.

Contact (relationship) There is a connection between things in the real world. There are two types of relations: one is the relationship between the attributes of the constituent entities within the entity, and the other is the relationship between the entities. When considering a relationship within an entity, the attribute is viewed as an entity. In general, the links between two entities can be divided into three types:

(1) One-to-one (1:1) contact if, for each entity in entity set A, there is at most a unique entity in entity set B associated with it, and vice versa, the entity set A is associated with the entity set B in the same way as 1:1.

(2) One-to-many (1:n) contact If there are n entities (n≥0) in entity set B associated with each entity in entity set A, in contrast, for each entity in entity set B, there is at most one entity in entity set a associated with it, and entity set A is associated with entity set B, which is recorded as 1:n. Accordingly there are many pairs of one (N:1) contacts, which are essentially a one-to-many linkage reversal. The same definition is similar to many links, no longer repeat.

(3) Many-to-many (m:n) contact if for each entity in entity set A, there are N entities in entity set B (n≥0) associated with it, and conversely, for each entity in entity set B, there are also m entities (m≥0) in entity set a corresponding to it, it is said that entity set A and entity set B have many-to-many connections, which are recorded as m:n. In essence, many-to-many linkages are any kind of connection. In addition, there can be various connections between entities within the same entity set. The most commonly used method of representation of conceptual models is the entity-one contact method (entity-relationship approach), referred to as the E-r method. The method was proposed by P.p.s.chen in 1976. The E-r method uses a e-r diagram to describe a conceptual model of an organization. Here are just a few highlights of the e-r chart. In the E-r diagram:

(1) The rectangle box represents the entity set, and the name of the entity type is written in the box.

(2) An ellipse box is used to represent the attribute of the entity, and the entity frame and its property frame are connected with a forward edge.

(3) Using a diamond box to indicate the relationship between the entities, the box is written in the contact name, with the non-edge of the diamond box and its related entity box linked together, in the next marked the type of contact. If the contact also has attributes, the property box and the Diamond box are also connected with the non-forward edge.

Featured Articles:

Prepare for 2014 soft exams! Featured Video Tutorials (Comprehensive review + experience sharing + pre-test sprint)

2014 Computer Soft test "network management" Knowledge point "Summary"

2014 Soft test network administrator exam exercises and Answers "Summary"

2014 Soft test programmer-written test-pre-Test practice Questions "Summary"

2014 Soft test programmer-Regular exam Knowledge point review Notes "Summary"

2014 Soft Test-Information technology handlers-simulation questions and Answers "Summary"

2014 Soft test-Information Processing technician-Exam Guidance "Summary"

For a soft exam online video tutorial, please click:

http://edu.51cto.com/course/courseList/id-44.html  

650) this.width=650; "src=" http://s3.51cto.com/wyfs02/M02/3D/9F/wKioL1PE_n3z629yAACXIHScsJM092.jpg "alt=" Wkiol1pe _n3z629yaacxihscsjm092.jpg "style=" padding:0px;margin:0px;vertical-align:top;border:none; "/>