Several criteria can be used to classify DBMSs. The first is the data model on which the DBMS is based. The main data model used in many current commercial DBMSs is the relational data model, and the systems based on this model are known as SQL systems. The object data model has been implemented in some commercial systems but has not had widespread use. Recently, so-called big data systems, also known as key-value storage systems and NOSQL systems, use various data models: document-based, graph-based, column-based, and key-value data models. Many legacy applications still run on database systems based on the hierarchical and network data models
The relational DBMSs are evolving continuously, and, in particular, have been incorporating many of the concepts that were developed in object databases. This has led to a new class of DBMSs called object-relational DBMSs. We can categorize DBMSs based on the data model: relational, object, object-relational, NOSQL, key-value, hierarchical, network, and other.
Some experimental DBMSs are based on the XML (eXtended Markup Language) model, which is a tree-structured data model. These have been called native XML DBMSs. Several commercial relational DBMSs have added XML interfaces and storage to their products
Homogeneous DDBMSs use the same DBMS software at all the sites, whereas heterogeneous DDBMSs can use different DBMS software at each site. It is also possible to develop middleware software to access several autonomous preexisting databases stored under heterogeneous DBMSs. This leads to a federated DBMS (or multidatabase system), in which the participating DBMSs are loosely coupled and have a degree of local autonomy
The XML model has emerged as a standard for exchanging data over the Web and has been used as a basis for implementing several prototype native XML systems. XML uses hierarchical tree structures. It combines database concepts with concepts from document representation models. Data is represented as elements; with the use of tags, data can be nested to create complex tree structures. This model conceptually resembles the object model but uses different terminology. XML capabilities have been added to many commercial DBMS products.