Question 2: Why should finance establish a mathematical model or not? Analysis data does not need mathematical model fitting, can't you guess out of thin air?
Question 3: Why is the data passed in when creating a model object 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 ...
Question 4: What are the three main data models in the process of 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?" It shouldn't be a short answer, so I'll do a few lines 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, and it is enough to understand these.
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 different models are used 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 how data is represented and accessed in the system, as well as how it is stored and accessed 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, 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 relationships are represented by relationships; The retrieval results of data are also relations); 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 establishment 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 the performance, users' query requests must be optimized, which increases the difficulty of developing database management system.
Question 5: 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 create a character model (just like peeling an onion, although it will shed tears, the onion tastes good)? Yes, we need to strip the souls of users, and then give them flesh and blood and put on coats (demographic characteristics). In this way, we will feel that users are around, vivid and impressive. It's not enough just to peel off the skin. We also need to summarize the classification, understand the goals, viewpoints and behaviors of users, and find the similarities and differences between users. According to the different research types and analysis methods of users, there are three methods to create role models: qualitative persona, quantitative verification qualitative persona and quantitative persona. Combined with the user role model project of Alibaba Chinese website transaction line, this paper analyzes the following creation methods: There are also many research methods, and the commonly used methods are: 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 creating 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? Segmentation groups 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 parameters of the 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 learn when creating persona models: 1. CRM data analysis binds the user's history and value to his questionnaire, looking for internal relations to better define or describe personas. It includes three types of data: 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 decisive for the creation 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. Browse the user's >>
Question 6: 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-keeping system (this definition emphasizes that a database is a * * * body of several records). Another example is a database, which is the * * * body of related data that people store together in a certain organizational way in order to solve specific tasks (this definition focuses on the organization of data). Moreover, the database is a data warehouse. Of course, although this statement is vivid, it is not rigorous. Strictly speaking, a database is a warehouse that organizes, stores and manages data according to the data structure. In the daily work of economic management, it is often necessary to put some relevant data into such a warehouse and process 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, it is also necessary to establish a large number of such databases to realize computer automatic management of finance, warehouse and production.
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 contains one 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 enjoyment 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. In particular, the microcomputer relational database management system dBASELL, which appeared in recent years, has the advantages of intuitive operation, flexible use, convenient programming, wide environmental adaptability (common 16-bit computers, such as IBM/PC/XT, domestic Great Wall 0520, etc., can run all kinds of software) and strong data processing ability. Database is being 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 connected 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 find someone's phone number conveniently, names and numbers are arranged in dictionary order, and the corresponding phone number is followed by the name. In this way, if you want to find a person's phone number (assuming that the first letter of his name is Y), you only need to find those names that start with Y. In this example, *** D of the data is the name and phone number, and the relationship R between them is arranged in dictionary order, and its 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 (that is, the connection and organization of data) and the relationship between data. & gt
Question django, what's the use of creating a data model?
This model has two functions.
On the one hand, decide the structure of the created * database *
What fields are there, what data type is each field, and can it be null=True?
On the other hand, it determines how the program operates the data in the database.
URL type, when * web page input *, you need to check whether the conditions of hyperlink are met.
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.
Rather than constraining the structure of the data.
In a word, blank is a restriction on user input, and null is a restriction on program/database.
Question 8: How to do empirical analysis? ! What data do you need? ! How to build a model or something? ! 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 tube with preheating function, even the ordinary 3-5 yuan lamp tube on the market can be used for 5 years.
Question 9: Why can't Excel20 13 check "Add this data to the data model" when creating a PivotTable report? 10, please check whether there are merged cells in the first line, and blank cells merge cells. If there are any above contents, there may be errors.