Simply put, what is a data warehouse?

A general database is an operational data storage tool, such as a database of the Course Selection System of a school. The course selection system is built and managed by the Academic Affairs Office. It is mainly used for Course Arrangement and Course Selection of students, you can add, modify, delete, and query course information in the course selection system. You can also operate the course selection information in the Course Selection System. Because the database is intended for selecting courses, it records information about the course arrangement and course selection. The database model is simplified as shown in:

Unlike normal operating databases, data warehouses are mainly for query operations, and the queried data comes from multiple database systems. Take a school as an example. In addition to the course selection system, the school also has a Human Resources System (HR system) used to manage teachers' information and student status information.

The HR system is built and managed by the Personnel Department and mainly stores information related to teachers. Therefore, the system does not contain student information, as shown in the system database model:

The student status management system is built and managed by the student office. The database stores basic information such as the student's degree, graduation destination, and family status, and stores the student's exam scores for each semester. Because the system focuses on students, there is no instructor information, as shown in the system database model:

Because different systems are built and managed by different departments and personnel, different information is stored. If you need to perform a joint query of data, and the queried data is stored in different databases, then, we need to integrate the information of these three system databases into a data model to form a data warehouse for joint query of the instructor information, student information, and course selection information, shows the data warehouse model:

For example, to check whether a teacher's training has an impact on the performance of the students taught by the teacher, so as to determine whether the next training for the teacher can be completed only in the data warehouse, because the instructor training information and student scores are stored in different systems and are independent of each other, it is impossible to query a separate database.

Compared with General databases, data warehouses also record the data change history. Data modification in the database is generally performed directly on the Data row, so only the latest data results are recorded, and the data warehouse records the changes of data on a daily basis, store the data before and after the change in the data warehouse for time series analysis.

Taking the three database and data warehouse of the school as an example, the basic information of each teacher is recorded in the HR system, one of which is the title of the teacher. When the instructor title changes, you only need to modify the instructor title field in the HR database. If you want to query the average score of the Students taught by a professor with different titles from the first lecturer to the present, you need to find out the change time of the instructor's title from the data warehouse, and then calculate the average score of the Students taught by the instructor in each time period, however, this query cannot be implemented in the HR database.