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Sixth Normal Form
Sixth normal form (6NF) is a normal form used in relational database normalization which extends the relational algebra and generalizes relational operators (such as join) to support interval data, which can be useful in temporal databases. The term 6NF has historically also been used to refer to another normalization degree, which today is more commonly known as domain-key normal form (DKNF) (see Other meanings). Definition Christopher J. Date and others have defined sixth normal form as a normal form, based on an extension of the relational algebra. Relational operators, such as ''join'', are generalized to support a natural treatment of interval data, such as sequences of dates or moments in time, for instance in temporal databases. Sixth normal form is then based on this generalized join, as follows: A relvar R ableis in sixth normal form (abbreviated 6NF) if and only if it satisfies no nontrivial join dependencies at all — where, as before, a join dependency i ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Database Normalization
Database normalization is the process of structuring a relational database in accordance with a series of so-called '' normal forms'' in order to reduce data redundancy and improve data integrity. It was first proposed by British computer scientist Edgar F. Codd as part of his relational model. Normalization entails organizing the columns (attributes) and tables (relations) of a database to ensure that their dependencies are properly enforced by database integrity constraints. It is accomplished by applying some formal rules either by a process of ''synthesis'' (creating a new database design) or ''decomposition'' (improving an existing database design). Objectives A basic objective of the first normal form defined by Codd in 1970 was to permit data to be queried and manipulated using a "universal data sub-language" grounded in first-order logic. An example of such a language is SQL, though it is one that Codd regarded as seriously flawed. The objectives of normalization ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Fifth Normal Form
Fifth normal form (5NF), also known as projection–join normal form (PJ/NF), is a level of database normalization designed to remove redundancy in relational databases recording multi-valued facts by isolating semantically related multiple relationships. A table is said to be in the 5NF if and only if every non-trivial join dependency in that table is implied by the candidate keys. It is the final normal form as far as removing redundancy is concerned. A 6NF also exists, but its purpose is not to remove redundancy and it is therefore only adopted by a few data warehouses, where it can be useful to make tables irreducible. A join dependency * on R is implied by the candidate key(s) of R if and only if each of A, B, …, Z is a superkey for R. The fifth normal form was first described by Ronald Fagin in his 1979 conference paper ''Normal forms and relational database operators''. Example Consider the following example: The table's predicate is: products of the type designa ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Fourth Normal Form
Fourth normal form (4NF) is a normal form used in database normalization. Introduced by Ronald Fagin in 1977, 4NF is the next level of normalization after Boyce–Codd normal form (BCNF). Whereas the second, third, and Boyce–Codd normal forms are concerned with functional dependencies, 4NF is concerned with a more general type of dependency known as a multivalued dependency. A table is in 4NF if and only if, for every one of its non-trivial multivalued dependencies ''X'' \twoheadrightarrow ''Y'', ''X'' is a superkey—that is, ''X'' is either a candidate key or a superset thereof."A relation schema R* is in fourth normal form (4NF) if, whenever a nontrivial multivalued dependency X \twoheadrightarrow Y holds for R*, then so does the functional dependency X → A for every column name A of R*. Intuitively all dependencies are the result of keys." Multivalued dependencies If the column headings in a relational database table are divided into three disjoint groupings ''X'', ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Second Normal Form
Second normal form (2NF), in database normalization, is a normal form. A relation is in the second normal form if it fulfills the following two requirements: # It is in first normal form. # It does not have any non-prime attribute that is functionally dependent on any proper subset of any candidate key of the relation (i.e. it lacks partial dependencies). A ''non-prime attribute of a relation'' is an attribute that is not a part of any candidate key of the relation. Put simply, a relation (or table) is in 2NF if: # It is in 1NF and has a single attribute unique identifier (UID) (in which case every non key attribute is dependent on the entire UID), or # It is in 1NF and has a multi-attribute unique identifier, and every regular attribute (not part of the UID) is dependent on ''all attributes'' in the multi-attribute UID, not just one attribute (or part) of the UID. If any regular (non-prime) attributes are predictable (dependent) on another (non-prime) attribute, that is ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
First Normal Form
First normal form (1NF) is the simplest form of database normalization defined by English computer scientist Edgar F. Codd, the inventor of the relational database. A Relation (database), relation (or a Table (database), ''table'', in SQL) can be said to be in first normal form if each field is ''atomic'', containing a single value rather than a set of values or a nested table. In other words, a relation complies with first normal form if no attribute domain (the set of values allowed in a given column) has relations as elements. Most relational database management systems, including standard SQL, do not support creating or using table-valued columns, which means most relational databases will be in first normal form by necessity. Otherwise, normalization to 1NF involves eliminating nested relations by breaking them up into separate relations associated with each other using foreign keys. This process is a necessary step when moving data from a non-relational (or NoSQL) database, ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Temporal Database
A temporal database stores data relating to time instances. It offers temporal data types and stores information relating to past, present and future time. Temporal databases can be uni-temporal, bi-temporal or tri-temporal. More specifically the temporal aspects usually include valid time, transaction time and/or decision time. * Valid time is the time period during or event time at which a fact is true in the real world. * Transaction time is the time at which a fact was recorded in the database. * Decision time is the time at which the decision was made about the fact. Used to keep a history of decisions about valid times. Types Uni-temporal A uni-temporal database has one axis of time, either the validity range or the system time range. Bi-temporal A bi-temporal database has two axes of time: * Valid time * Transaction time or decision time Tri-temporal A tri-temporal database has three axes of time: * Valid time * Transaction time * Decision time This approach intro ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Database Theory
Database theory encapsulates a broad range of topics related to the study and research of the theoretical realm of databases and database management systems. Theoretical aspects of data management include, among other areas, the foundations of query languages, Computational complexity theory, computational complexity and expressive power (computer science), expressive power of queries, finite model theory, database design theory, dependency theory (database theory), dependency theory, foundations of concurrency control and database recovery, deductive databases, temporal database, temporal and spatial databases, real-time databases, managing uncertain data and probabilistic databases, and Web data. Most research work has traditionally been based on the relational model, 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 models, or, more recently, graph ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Data Vault Modeling
Datavault or data vault modeling is a database modeling method that is designed to provide long-term historical storage of data coming in from multiple operational systems. It is also a method of looking at historical data that deals with issues such as auditing, tracing of data, loading speed and resilience to change as well as emphasizing the need to trace where all the data in the database came from. This means that every row in a data vault must be accompanied by record source and load date attributes, enabling an auditor to trace values back to the source. The concept was published in 2000 by Dan Linstedt. Data vault modeling makes no distinction between good and bad data ("bad" meaning not conforming to business rules). This is summarized in the statement that a data vault stores " a single version of the facts" (also expressed by Dan Linstedt as "all the data, all of the time") as opposed to the practice in other data warehouse methods of storing "a single version of ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Candidate Key
A candidate key, or simply a key, of a relational database is any set of columns that have a unique combination of values in each row, with the additional constraint that removing any column could produce duplicate combinations of values. A candidate key is a minimal superkey, i.e., a superkey that does not contain a smaller one. Therefore, a relation can have multiple candidate keys, each with a different number of attributes. Specific candidate keys are sometimes called ''primary keys'', ''secondary keys'' or ''alternate keys''. The columns in a candidate key are called prime attributes, and a column that does not occur in any candidate key is called a non-prime attribute. Every relation without NULL values will have at least one candidate key: Since there cannot be duplicate rows, the set of all columns is a superkey, and if that is not minimal, some subset of that will be minimal. There is a functional dependency from the candidate key to all the attributes in the relatio ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Third Normal Form
Third normal form (3NF) is a database schema design approach for relational databases which uses normalizing principles to reduce the duplication of data, avoid data anomalies, ensure referential integrity, and simplify data management. It was defined in 1971 by Edgar F. Codd, an English computer scientist who invented the relational model for database management. A database relation (e.g. a database table) is said to meet third normal form standards if all the attributes (e.g. database columns) are functionally dependent on solely a key, except the case of functional dependency whose right hand side is a prime attribute (an attribute which is strictly included into some key). Codd defined this as a relation in second normal form where all non-prime attributes depend only on the candidate keys and do not have a transitive dependency on another key. A hypothetical example of a failure to meet third normal form would be a hospital database having a table of patients which ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
