A model is an abstraction of the real world. In database technology, the model representing entity types and the relationship between entity types is called "data model". Data model is a teaching framework of database management, which is used to describe the concept and definition of a group of data, including three aspects: 1, conceptual data model: this is number-oriented. ...

Why should finance establish a mathematical model?

Or what? Analysis data does not need mathematical model fitting, guess out of thin air?

Why should the data passed in when creating a model object be rewritten later?

This is called object-relational database mapping. The principle of Hibernate, the core part, Object Relational Mapping (ORM) provides a conceptual and easy-to-understand method to model data. ORM method is based on three core principles: simplicity: modeling data in the most basic form. Communication: The database structure is recorded in a language that anyone can understand. precise ...

What are the three main data models in database development?

Generally, a database corresponds to a data model, so the correct formulation is: What are the main data models in the database?

It is estimated that you will take the Introduction to Database next, hehe! In my experience, if you test the database, you can have at most one multiple-choice question or fill-in-the-blank question, even if you test "What are the characteristics of these models?" I don't think it's a short answer. I'll take some dry goods to test you. After all, "database" is not a "college ideological and political course".

This should be the knowledge in the introduction of the first chapter of Introduction to Database (fourth edition). The introduction is the introduction in the basic chapter, which should be said to be a guiding concept. The practical application of these concepts is carried out in the following chapters, so it is enough to understand them.

What are the main data models?

A: Model: The simulation and abstraction of the characteristics of an object in the real world.

Understand; Understanding

Two types of data models:

Data models are divided into two categories (belonging to two different levels and using different models when developing and using databases).

① Conceptual model, also known as information model, is used to model data and information from the user's point of view, and is used for database design.

② logical model and physical model,

Logical models mainly include: grid model, hierarchical model, relational model, object-oriented model and so on. Modeling data from the perspective of computer system to realize DBMS.

Physical model is the lowest abstraction of data, which describes the representation and access methods of data in the system and the storage and access methods on disk or tape.

Conceptual model: the basic concept of the information world.

Usage: the language of communication between database designers and users. Therefore, this understanding is enough; But take the E-R diagram!

The most commonly used data models: non-relational model, hierarchical model and grid model; Relational model; Object-oriented model and object-relational model.

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grasp

Hierarchical model: use "tree structure" to represent various entities and the relationship between entities.

Features: the parent node of the node is unique; Only one-to-many entity contact can be handled directly; Each record type can define a sort field, also known as: code field; The value of any record can only show its full meaning according to its path; The child record value cannot exist independently of the parent record value.

Grid model: * * * meets the following two conditions: ① multiple nodes are allowed to have no parent node; ② A node can have multiple parent nodes.

Features: advantages: it can describe the real world more directly, for example, a node can have multiple parent nodes; It has good performance and high access efficiency.

Disadvantages, the structure is more complex, and with the expansion of the application environment, the structure of the database is more and more complex, which is not conducive to the end user to master; DDL and DML languages are complex and difficult for users to use.

Relational model: In the view of users, the logical structure of data in relational model is a two-dimensional table, which consists of rows and columns.

Features: advantages, based on strict data concepts; The concept is single (entities and various connections are represented by relationships; The retrieval results of data are also relevant); The access path of relational model is transparent to users (with higher data independence and better security; Simplifies the work of programmers and the development and creation of databases).

Disadvantages: the access path is transparent to users, which leads to the query efficiency is often not as good as that of non-relational databases; In order to improve efficiency, users' query requests must be optimized, which increases the difficulty of developing database management system.

Why do you need a user role model?

Recently, I am also doing some activity pages and mobile games, and gradually realize the importance of role models. Role model is a street lamp when designing products, and it is a design reference for product managers and interaction designers. Is it peeling to set an example (just like peeling an onion, although it will shed tears, the onion tastes good)? Yes, we need to strip users of their souls, then give them flesh and blood and put on their coats (demographic characteristics). In this way, we will feel that users are around, vivid and impressive. Peeling is not enough. We also need to summarize the classification, understand the goals, viewpoints and behaviors of users, and find the differences and commonalities between users. According to the different methods of user research and analysis, there are three methods to establish role model: qualitative role, quantitative verification qualitative role and quantitative role. Combined with the user role model project of Alibaba Chinese website transaction line, the following methods are analyzed: There are many research methods, such as questionnaire survey, user interview, on-site observation, usability test, data analysis and website traffic/log analysis. In the transaction line project, interviews, questionnaires and data analysis are conducive to discovering users' goals and opinions; Field observation and website traffic/log analysis are helpful to understand users' behavior. In the process of establishing a role model, the following problems are often encountered: 1. How to use data to subdivide? How to see the law of data? Find out the latitude difference from the data and find out all the related factors that cause this difference. 2. How to design the questionnaire? What is the latitude? According to the whole transaction process-order-management-payment-logistics and product dimensions (considering the actual operation process of users). 3. How to write an in-depth interview outline? What do you know about users? Please refer to the user role division dimension questionnaire. 4. How to conduct CRM analysis? See related topic 5. How to do crosstab analysis? See related topic 6. How to segment users? Generally speaking, it is subdivided according to the user's goals, according to the use cycle, and according to the combination of behavior and opinions. In the transaction line persona project, according to the goals, behaviors and viewpoints that drive users, role segmentation is the key factor leading to differences, such as different commodity sources and different shopping motives. 7. How to preliminarily test the subdivision latitude? Subgroups can explain the known key differences, such as: different housing goals (second-hand housing users and new housing users), and can explain the differences in keyword search and use); Subdivision groups should play a decisive role in determining functional design, interactive design and sketch. 6. What are the methods of quantitative verification? Data cross-Tab analysis (CRM analysis, quantitative questionnaire, website traffic/log analysis), statistical analysis. 7. What characteristics do personas need? Referring to the argument of role model, personas are driven by goals, behaviors and opinions, rather than some simple demographic characteristics. 8. Use of persona model? Develop new functions and functional improvements (understand user needs) and interact with design details (understand user habits). Related topics to be studied in setting an example: 1. CRM data analysis links the user's history and value with his questionnaire to find the internal relationship, so as to better define or describe the role. It includes: transaction records, financial data and demographic information. Transaction records show which products or services users have purchased and how often they have purchased them, which will strongly affect the goals and behaviors of the website and can be used as one of the bases for user segmentation. Financial data, using numbers to measure the financial value of different personas, can also help determine the priority level of each persona. Financial data can be associated with user surveys. Demographic information is not very decisive for the establishment of personas, which are driven by goals, behaviors and opinions. 2. There are two ways to analyze website traffic: a. Find the decisive behavior pattern and analyze the data, trying to link the data results with the behaviors of the subdivided groups. B. Bind the click stream of an individual user with the questionnaire he replied, and make further detailed analysis. Explore the user's ......

Why use a database?

When people describe this concept from different angles, they have different definitions (descriptive, of course). For example, a database is called a "record storage system" (this definition emphasizes that a database is a * * * body of several records). For another example, a database is "a * * body of related information that people store together in a certain organizational way in order to solve a specific task" (this definition focuses on the organization of information). More importantly, the database is called "data warehouse". Of course, although this statement is vivid, it is not rigorous. Strictly speaking, a database is a "warehouse for organizing, storing and managing data according to data structure". In the daily work of economic management, it is often necessary to put some relevant information into such a "warehouse" and handle it according to the needs of management. For example, the personnel department of enterprises and institutions often stores the basic information of employees (job number, name, age, gender, place of origin, salary, resume, etc.). ) In Table 20.6.3, it can be regarded as a database. With this "data warehouse", we can query the basic situation of an employee at any time as needed, and we can also query the number of employees whose salary is within a certain range, and so on. If all these tasks can be done automatically on the computer, then our personnel management can reach a very high level. In addition, in financial management, warehouse management and production management, many such "databases" need to be established, so as to realize the automatic management of finance, warehouse and production by computers.

J. Martin gave a relatively complete definition of database: database is a * * body of related data stored together, which is structured, without harmful or unnecessary redundancy and serves many applications; The storage of data is independent of the program that uses it; Inserting new data into the database, modifying and retrieving original data can be done in a universal and controllable way. When there are several completely separated databases in a system, the system includes a "database * * *".

? Advantages of database

Using database can bring many benefits: for example, it reduces the redundancy of data, thus greatly saving the storage space of data; Realize the full sharing of data resources and so on. In addition, database technology also provides users with a very simple means of use, so that users can easily write database applications. Especially in recent years, the microcomputer relational database management system dBASELL has the advantages of intuitive operation, flexible use, convenient programming and wide environmental adaptability (common 16-bit computers, such as IBM/PC/XT, domestic Great Wall 0520, etc. ) and strong data processing ability. Database is widely used in China, and it will become a powerful tool for economic management.

Database realizes data storage, management and use through DBMS- database management system software. DBASELL is a database management system software.

? Database structure and database type

Databases are usually divided into three types: hierarchical database, network database and relational database. Different databases are linked and organized according to different data structures.

1. data structure model

(1) data structure

The so-called data structure refers to the organizational form of data or the relationship between data. If d represents data and r represents the relationship between data objects, then DS = (d, r) is called data structure. For example, there is a telephone directory in which the names and corresponding telephone numbers of N people are recorded. In order to query someone's telephone number conveniently, names and numbers are arranged in dictionary order, and the corresponding telephone number is followed by the name. In this way, if you want to query someone's phone number (assuming that the first letter of his name is Y), you only need to query those names that start with Y. In this example, *** D of the data is the name and phone number, and the contacts R between them are arranged in dictionary order, and the corresponding data structure is DS = (d, R), which is an array.

(2) the type of data structure

Data structure is divided into logical structure and physical structure of data. The logical structure of data is to observe and analyze data from a logical point of view (that is, the way of communication and organization between data) ......

Django, what's the use of building a data model?

Django, what's the use of building a data model?

This model has two functions.

On the one hand, determine the structure of * database *

What are the areas? What is the data type of each field? Can it be null=True?

On the other hand, it determines how the program operates the data in the database.

The URL type, when * web page input *, need to check whether meet the hyperlink conditions.

Blank=True determines whether data can be blank when entering on a web page.

When data is written in a program, it is not checked.

Is not a constraint on the data structure.

In a word, blank is a restriction on user input, and null is a restriction on program/database.

How to do empirical analysis? ! What information do you need? ! How to build a model or something ! 5 points

The service life of the lamp depends on the ballast. If the ballast is not good, even the best lamp won't last long. If the valve flow device is matched with the lamp with preheating function, even the ordinary 3-5 yuan lamp on the market can last for 5 years.

Why can't Excel20 13 check "Add this data to the data model" when creating a PivotTable report? 10 point

Please check whether there are merged cells in the first row, and blank cells merge cells. If there are any above contents, there may be errors.

Why introduce data model into database system?

Key technical problems to be solved in the research and development of multimedia database;

Multimedia data model

Multimedia data model mainly adopts the extension of relational data model and object-oriented design method. Because the traditional relational model is difficult to describe multimedia information and define multimedia data objects, at present, in addition to introducing abstract data types, semantic models are more used in the expansion of relational models. Relational model mainly describes the structure of data, while semantic model mainly expresses the semantics of data. Semantic model is higher than relational model, which can be used as the basis of the former. The current research shows that it is a good method to describe and establish multimedia data model with object-oriented method. The main concepts of object-oriented include object, class, method, message, encapsulation and inheritance, which can describe complex multimedia information conveniently.

Data compression and decompression

Due to the large amount of multimedia data such as audio and video, storage and transmission require a lot of space and time, so data compression and coding should be considered, and the compression method should consider complexity, implementation speed and compression quality.

Storage management and access method of multimedia data

At present, paging management, B+ tree and Hash method are commonly used. Multimedia database should also introduce content-based retrieval method, vector space model information index retrieval technology, over-retrieval technology and intelligent index technology.

D. copy multimedia information and a good user interface

Multimedia database should provide multimedia host language call, and also provide various editing and conversion functions of sound, image, graphics and dynamic video.

E. Decentralization technology

Multimedia data communication requires high network bandwidth and needs corresponding high-speed network. In addition, the problems of data integration, query, scheduling and sharing of heterogeneous multimedia data languages need to be solved.

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