Describing the logical structure and characteristics of localized data in a database is the outer schema.

The schema structure of a database system type, a description of the structure and attributes of a particular type of data. Value, a specific assignment of the type. A description of the logical structure and characteristics of all the data in the database, a public **** view of the data for all users, synthesizing the needs of all users, a database has only one schema.

The status of the schema: is the middle layer of the database system schema structure, with the physical storage details of the data and hardware environment has nothing to do with the specific applications, development tools and high-level programming language. The link between the data, data-related security, integrity requirements.

A description of the logical structure and characteristics of localized data used by database users (including application programmers and end users), a database user's view of the data, a logical representation of the data related to an application.

The status of the outer schema: the relationship between the schema and the application between the schema and the outer schema, the outer schema is usually a subset of the schema, a database can have more than one outer schema, reflecting the application needs of different users, the way of looking at the data as well as the requirements for data confidentiality. For the same data in the schema, the structure, type, length, and confidentiality level can be different in the outer schema.

Database development status

In the history of database development, the database has gone through the development of hierarchical databases, mesh databases and relational databases and other stages of the development of database technology in all aspects of the rapid development. In particular, relational databases have become the most important member of the current database products, since the 80's, almost all database vendors new database products support relational databases, even if some non-relational database products almost all have interfaces that support relational databases.

This is mainly because the traditional relational database can be a better solution to the problem of managing and storing relational data, with the development of cloud computing and the arrival of the era of big data, relational databases are increasingly unable to meet the needs of the main reason is that more and more semi-relational and non-relational data need to use the database for storage and management. At the same time, the emergence of new technologies such as distributed technology also put forward new requirements for database technology, so more and more non-relational databases began to appear.