What is the difference between a document database and a traditional relational database?

There are significant differences between document database and traditional relational database. Relational databases usually store data in independent tables, which are defined by program developers, and a single object may be scattered in several tables. For a given object, a document database stores all its information in a single instance of the database, and each stored object can be different from any other object. This simplifies the process of loading objects into the database and usually eliminates the need for similar schemes, such as object relational mapping. Document database is different from relational database, which is based on relational model, while document database adopts semi-structured model, with no separation between data and schema, and the number of structures used depends on the target purpose. In semi-structured data, entities belonging to the same class can have different attributes, even if they are grouped together, the order of attributes is not important.