A database is an organised collection of data that can be queried and updated in real time. Modern database systems are mainly relational database management systems (RDBMS) or object-relational database management systems (ORDBMS). A RDBMS is a database management system based on a relational mathematical model comprising set theory and predicate logic. An ORDBMS is similar to an RDBMS but is implemented using an object-oriented database model with objects, classes and inheritance directly supported by the database schema and query language. Various RDBMS implementations exist, the most prevalent ones being Microsoft SQL Server and MySQL. The most common ORDBMS in use is the Oracle Database.