Relational database management systems are the best known type of database managers today, and certainly the most successful if viewed in terms of revenue and brand profile of the vendors. Commonly used relational database management systems include Microsoft SQL Server, Oracle, MySQL, Sybase, DB/2 and PostgreSQL.
The core defining characteristic of a relational database management system is the ability to join two data sets together by matching on common column values (known as an equi-join or other predicates (known as a t-join). Relational databases are solely queried through a query language interface - typically SQL, although other query languages have been used.
The logical model behind relational databases is known as relational algebra, originally described in Edgar Codd's 1970 paper A relational model for large shared data banks. However, modern SQL dialects extend substantially on the expressiveness of this model.