Database schema definition Language

The database schema definition language DDL (Data Description Language) is the language used to describe the real-world entities to be stored in the database. A database schema contains descriptive definitions for all entities in the database. These definitions include the structure definition, the operation method definition, and so on.

The pattern of DDL description, which must be compiled by computer software and converted into a format convenient for computer storage, querying and manipulating, is called a schema compiler.

Schema compiler processing pattern definitions mainly produce two types of data: data dictionary and data type and structure definition.

The data dictionary and the internal structure information of the database are the basis of creating the database corresponding to the pattern, based on which the logical structure of each database is created; Access to database data, queries based on schema information determine the way and type of data access, and the relationship between data and the integrity of data constraints.

A data dictionary is a representation of the internal information of a pattern, and the data dictionary is stored differently from different DBMS.

The definition of data type and structure means that when an application is connected to a database, the application needs to understand the data types and structures that are generated and extracted. is the data type and structure definition of the user workspace that is provided for the various host languages, which is consistent with the logical structure of the user workspace and the database, and reduces the data conversion process, which is typically defined with a single header file.

There are usually two ways to define a database schema: interactively define patterns and describe text definition patterns through data Description Language DDL.