computer mode structure has three kinds, namely: external mode, mode and internal mode, of which external mode is also generally called user sub-mode.
Definition: Also known as Subschema or User mode, it is the database user (including application programmers and end users) The description of the logical structure and characteristics of the local data seen and used is a data view of the database user and is a logical representation of the data associated with an application.
理解: 1 A database can have multiple external modes; 2 external mode is the user view; 3 external mode is a powerful measure to ensure data security.
Definition: Also known as the logical mode, it is a description of the logical structure and characteristics of the entire data in the database, and is a common data view of all users.
理解: 1 A database has only one schema; 2 is a view of database data at a logical level; 3 Database schema is based on a certain data model; 4 When defining a schema, not only the logical structure of the data (such as data) Record which data items are composed, the name, type, value range, etc. of the data item, and define the security and integrity requirements related to the data, and define the relationship between the data.
三, Internal Schema
: Also known as Storage Schema, it is a description of the physical structure and storage of data, and is how data is represented inside the database (for example, records). The storage method is sequential storage, storage according to B-tree structure or hash method; in what way index is organized; whether data is compressed and stored, whether it is encrypted; what is the definition of data storage record structure).
理解: 1 A database has only one internal mode; 2 A table may consist of multiple files, such as data files and index files. It is a method for database organization system (DBMS) to effectively organize and manage data in the database. Its purpose is: 1 To reduce data redundancy and achieve data sharing; 2 In order to improve access efficiency and improve performance.