Level three schema structure in the database

Schema (schema) is a descriptive narrative of the logical structure and characteristics of all data in a database. Also known as conceptual mode or conceptual view. Views can be understood as a set of recorded values that the user or program ape sees and uses for the contents of the database.

The pattern is in the middle of a three-level structure. It is an abstract representation of the actual storage of the entire database, as well as an abstraction of the real world, a representation of the entire set of information content for an application environment (enterprise or unit) in the real world, as well as a combination of all individual user views, and therefore a common view of the user.

A pattern is a data model that is precisely defined using descriptive language of data. Three levels of the data pattern: the outer layer, the conceptual layer, and the inner layer. The three-level model structure of database system, no matter how the function of database management system changes, how the platform of operating system is different, how the data model is different, the characteristics of the three-level model structure of database system basically remain unchanged.

The external mode, also known as the user mode, is defined as a sub-mode in the traditional database system. A sub-pattern is a subset of conceptual patterns. The external mode defines the data by user view and can also be exported from the conceptual mode.

The conceptual model, also known as the logical mode, is the twisted band between the real world and the data storage. The concept pattern is the abstract and descriptive narration of the global data logical relationship, which is independent of the physical storage structure of the data, and is the common logical view of all the users after the database design is realized.

The internal model is also called the physical model, which describes how the data is stored and accessed in the database.

The DBMS provides an in-mode descriptive narrative language (internal schema DDL) to define internal patterns.

In the database system, the outer mode can have many, but the concept mode, inner mode can only have one.

The internal pattern is the representation of the actual storage of the entire database, and the conceptual pattern is an abstract representation of the actual storage of the entire database. The outer pattern is an abstract representation of a part of a conceptual pattern.

All of these patterns must be defined before the database is used.

The database management system provides the model descriptive narrative language DDL. Used to strictly describe the definition of all entities in a database. is compiled and stored in the database. The outer mode is associated with each detailed application and the Advanced programming language it uses.

The internal mode is related to the physical storage (operating system) and hardware of the data. The conceptual pattern is independent of the detailed application and physical environment. Is the logical representation of all data in the database.

The process of database design is to synthesize the external mode, produce the conceptual pattern, and produce the physical pattern according to the DBMS used.

The user view is then exported by the conceptual mode, which is the logical data that the user sees.

The three-level schema structure of the database is three levels of abstraction for the data.

Between these three levels of abstraction. In order to realize the transformation of data, the database management system must provide two layer mapping function, namely the mapping of the outer mode and the conceptual mode, the conceptual mode and the internal model.

The mapping between the external model and the conceptual model defines the corresponding relationship between the outside model and the conceptual model, usually giving descriptive narration in the external mode. The purpose of this layer mapping is to realize the independence of logical data. When the overall logical structure of the data changes, it is assumed that an external pattern remains unchanged, and that the mapping of the corresponding external and conceptual patterns needs to be changed. Thus the corresponding application of the external mode does not have to change.

The mapping of conceptual patterns and internal patterns exists in the conceptual layer, and the corresponding relationship between them is established by the DBMS. The role of this layer of mapping is to achieve the independence of physical data.

When the physical storage mode of a database changes, such as when changing storage devices and access methods. The concept pattern can be kept constant, and the mapping relationship between the corresponding internal pattern and the conceptual model needs to be changed. The application does not have to change.

Advantages of Level Three structure:

(1) Ensure the independence of the data. Separating the pattern from the inside mode ensures the physical independence of the data. The external mode and mode are separated to ensure the logical independence of the data.

(2) simplifies the user interface.

Write the application or type the command according to the external mode. Without needing to know the storage structure inside the database. User-friendly system is available.

(3) Facilitate data sharing.

In different external modes, multiple users can share the data in the system and reduce the data redundancy.

(4) To facilitate the security and confidentiality of data. In the external mode according to the requirements of operation, not to the limited data operation, to ensure the security of other data.

　　

Level three schema structure in the database