Database theory encapsulates a broad range of topics related to the study and research of the theoretical realm of database

Database

A database is an organized collection of data for one or more purposes, usually in digital form. The data are typically organized to model relevant aspects of reality , in a way that supports processes requiring this information...

s and database management system

Database management system

A database management system is a software package with computer programs that control the creation, maintenance, and use of a database. It allows organizations to conveniently develop databases for various applications by database administrators and other specialists. A database is an integrated...

s.

Theoretical aspects of data management include, among other areas, the foundations of query languages, computational complexity

Computational Complexity

Computational Complexity may refer to:*Computational complexity theory*Computational Complexity...

 and expressive power

Expressive power

In computer science, the expressive power of a language describes the ideas expressible in that language.For example, the Web Ontology Language expression language profile lacks ideas which can be expressed in OWL2 RL . OWL2 EL may therefore be said to have less expressive power than OWL2 RL...

 of queries, finite model theory

Finite model theory

Finite Model Theory is a subarea of model theory . MT is the branch of mathematical logic which deals with the relation between a formal language and its interpretations . FMT is a restriction of MT to interpretations of finite structures, i.e...

, database design theory, dependency theory

Dependency theory (database theory)

Dependency theory is a subfield of database theory which studies implication and optimization problems related to logical constraints, commonly called dependencies, on databases....

, foundations of concurrency control

Concurrency control

In information technology and computer science, especially in the fields of computer programming , operating systems , multiprocessors, and databases, concurrency control ensures that correct results for concurrent operations are generated, while getting those results as quickly as possible.Computer...

 and database recovery, deductive database

Deductive database

A Deductive database is a database system that can make deductions based on rules and facts stored in the database. Datalog is the language typically used to specify facts, rules and queries in deductive databases...

s, temporal

Temporal database

A temporal database is a database with built-in time aspects, for example a temporal data model and a temporal version of Structured Query Language.More specifically the temporal aspects usually include valid-time and transaction-time...

 and spatial database

Spatial Database

A spatial database is a database that is optimized to store and query data that is related to objects in space, including points, lines and polygons. While typical databases can understand various numeric and character types of data, additional functionality needs to be added for databases to...

s, real time database

Real time database

A real-time database is a processing system designed to handle workloads whose state is constantly changing . This differs from traditional databases containing persistent data, mostly unaffected by time. For example, a stock market changes very rapidly and is dynamic...

s, managing uncertain data and probabilistic database

Probabilistic database

A probabilistic database is an uncertain database in which the possible worlds have associated probabilities. Probabilistic database management systems are currently an active area of research...

s, and Web data.

Most research work has traditionally been based on the relational model

Relational model

The relational model for database management is a database model based on first-order predicate logic, first formulated and proposed in 1969 by Edgar F...

, since this model is usually considered the simplest and most foundational model of interest. Corresponding results for other data models, such as object-oriented or semi-structured model

Semi-structured model

The semi-structured model is a database model. In this model, there is no separation between the data and the schema, and the amount of structure used depends on the purpose.The advantages of this model are the following:...

s, or, more recently, graph data models and XML

XML

Extensible Markup Language is a set of rules for encoding documents in machine-readable form. It is defined in the XML 1.0 Specification produced by the W3C, and several other related specifications, all gratis open standards....

, are often derivable from those for the relational model.

A central focus of database theory is on understanding the complexity and power of query languages and their connection to logic

Logic

In philosophy, Logic is the formal systematic study of the principles of valid inference and correct reasoning. Logic is used in most intellectual activities, but is studied primarily in the disciplines of philosophy, mathematics, semantics, and computer science...

. Starting from relational algebra and first-order logic (which are equivalent by Codd's theorem

Codd's theorem

Codd's theorem states that relational algebra and the domain-independent relational calculus queries, two well-known foundational query languages for the relational model, are precisely equivalent in expressive power. That is, a database query can be formulated in one language if and only if it can...

) and the insight that important queries such as graph reachability are not expressible in this language, more powerful language based on logic programming

Logic programming

Logic programming is, in its broadest sense, the use of mathematical logic for computer programming. In this view of logic programming, which can be traced at least as far back as John McCarthy's [1958] advice-taker proposal, logic is used as a purely declarative representation language, and a...

 and fixpoint logic such as datalog

Datalog

Datalog is a query and rule language for deductive databases that syntactically is a subset of Prolog. Its origins date back to the beginning of logic programming, but it became prominent as a separate area around 1977 when Hervé Gallaire and Jack Minker organized a workshop on logic and databases...

 were studied. Another focus was on the foundations of query optimization

Query optimization

Query optimization is a function of many relational database management systems in which multiple query plans for satisfying a query are examined and a good query plan is identified. This may or not be the absolute best strategy because there are many ways of doing plans. There is a trade-off...

 and data integration

Data integration

Data integration involves combining data residing in different sources and providing users with a unified view of these data.This process becomes significant in a variety of situations, which include both commercial and scientific domains...

. Here most work studied conjunctive queries

Conjunctive query

In database theory, a conjunctive query is a restricted form of first-order queries. A large part of queries issued on relational databases can be written as conjunctive queries, and large parts of other first-order queries can be written as conjunctive queries....

, which admit query optimization even under constraints using the chase

Chase (algorithm)

The Chase is a simple fixpoint algorithm testing and enforcing implication of data dependencies in database systems. It plays important roles in database theory as well as in practice....

 algorithm.

The main research conferences in the area are the ACM Symposium on Principles of Database Systems (PODS) and the International Conference on Database Theory (ICDT).