Information about Relational Database
A relational database is a database that conforms to the relational model, and refers to a database's data and schema (the database's structure of how that data is arranged). Common usage of the term "Relational database management system" technically refers to the software used to create a relational database, but sometimes mistakenly refers to a relational database.
The term relational database was originally defined and coined by E.F. Codd.[1]
A relational database management system (RDBMS) is a system that manages data using the relational model. Frequently, the term "RDBMS" is inaccurately used as a generic label for the relational database concept. Most current RDBMSs (for example: MySQL, PostgreSQL, Oracle, Microsoft SQL Server, Ingres) deviate significantly from the relational model and are more accurately called SQL database management products. See current usage for an explanation of the requirements for a DBMS to fully support the relational model.
A relation is defined as a set of tuples that all have the same attributes. This is usually represented by a table, which is data organized in rows and columns. In a relational database, all of the data stored in a column should be in the same domain (i.e. data type). In the relational model, the tuples should not have any ordering. This means both that there should be no order to the tuples, and that the tuples should not impose an order of the attributes. Put differently, neither the rows nor the columns should have an order to them.
While this is the desired result, it is not universally achieved. The SQL standard requires columns to have a defined order. All data stored in a computer has to have an order, as the memory of a computer is linear. Also, when the data is returned, there must be an order in which the data is returned (because all transfer protocols are linear, and coincidentally enough, humans read in a linear fashion). The point here is that this order must never make a logical difference in the system. Frequently orders are imposed which impact performance, but they should never change the result of a query on the database. In practice, several of the DBMSs that are considered "relational" impose an order that makes a logical difference.
Constraints are a way of providing restrictions on the kinds of data that can be stored in the relations. These are usually defined (formally) in the form of expressions that result in a boolean value, indicating whether or not the constraint holds. Constraints are a way of implementing business rules into the database.
Under the strictest sense, constraints are not considered part of the relational database, but because of the integral role which they play in organizing data, they are usually considered part of the database.
A data domain (or usually just domain), is the set of possible values for a given attribute. Because it does constrain the values the data can hold, it could be considered a constraint, but because attributes must specify a domain, it could just be considered part of the relation's definition. Mathematically, a domain can be expressed as "all values for this attribute must be an element of the specified set."
A tuple usually represents some object and its associated data, whether that object is a physical object or a concept. A key is a kind of constraint which requires that the object, or critical information about the object, isn't duplicated. For example, a family might like to have a constraint such that no two people in the immediate family have the same name. If information about family members were stored in a database, a key could be placed over the family member's name. In a University, they have no such luxury. Each student is typically assigned a Student ID, which are used as keys for individual students stored in the school database. Keys can have more than one column, for example, a nation may impose a restriction that a province can't have two cities by the same name. So, when cities are stored in a relation, there would be a key defined over province and city name. This would allow for two different provinces to have a town called Springfield (because their province would be different), but not two cities with the same name in the same province. A key over more than one attribute is called a compound key. Theoretically, a key can even be over zero attributes. This would enforce that there cannot be more than one tuple in the relation.
Most relations have at least one key defined on it. Because a relation is defined in the relational model as being a set, it can't have duplicate rows. Some DBMSs don't enforce this. If a DBMS does enforce this, it means that there is always at least one key on each relation, namely the key involving all of the attributes of the relation.
A key could be defined formally by requiring that the cardinality of the relation should be equal to the cardinality of the relation projected over the columns of the key.
A key, in this context, refers to any set of attributes which uniquely span the relation. In particular, this is called a superkey. A candidate key is a minimal superkey, meaning that, none of the attributes in the key could be removed from the key, and still have that attribute set be a key. Many DBMSs have a concept of a primary key. The primary key (usually a candidate key) is the key most often used to identify a tuple. In some RDBMSs, the primary key of a base relvar is the storage key (sometimes clustered key), meaning that that is how the data is stored physically. If the value of the primary key is actual interesting data with logical ties to the data (like a name) for the tuple, it is called a natural key. If the key is generated and doesn't have any logical connection to the rest of the data in the tuple, it is called a surrogate key. Other candidate keys that were not chosen as the primary key are called alternate keys.
A foreign key is not a key by the previous definition. Rather, a foreign key is a reference to a key in another table. Meaning that the referencing tuple has, as part of its attributes, the values of a key in the referenced tuple that corresponds to the relationship.
A foreign key could be described formally as "For all tuples in the referencing relation projected over the referencing attributes, there must exist a tuple in the referenced relation projected over those same attributes such that the values in each of the referencing attributes match the corresponding values in the referenced attributes".
A transition constraint is a way of enforcing that the data doesn't enter an impossible state because of a previous state. For example, it shouldn't be possible for a person to change from being "married" to being "single, never married". The only valid states after "married" might be "divorced", "widowed", or "deceased".
A relvar is a "relation variable". In a relational database, all data are stored and accessed via relations. The data that are actually stored in the database are stored as relations. These relations are sometimes called "base relvars". This is equivalent to a "table". Other relvars do not have their data stored in them, but are a result of applying relational operations, to other relvars. These relvars are sometimes called "derived relvars", meaning that their information is derived from other sources. These are equivalent to "views" or "queries". Derived relvars are convenient in that though they may grab information from several relvars, it is presented externally as a single relvar for a simpler perspective. Also, it can be used as an abstraction layer.
Derived relvars are not always considered part of a relational database, partially because they are not essential to the functioning of the database.
A stored procedure is executable code that is associated with the database. Stored procedures usually store how to perform common operations, like inserting a tuple into a relation, or gathering statistical information about usage patterns. Frequently they are used as an application programming interface (API) for security or simplicity. These are usually written as Imperative programming code extending the Data Definition Language and/or the Data Manipulation Language for the DBMS.
Stored procedures are not always considered part of a relational database, partially because they are not essential to the functioning of the database.
An index is a way of providing quicker access to the data in a relational database. Indices can be created on any combination of attributes on a relation. Then when tuples in a relation need to be looked up, similar to how a book's index works, the index can be accessed. Rather than having to check all of the tuples, the index tells the DBMS where the tuple is. Indices are usually implemented via B+ trees.
Indices are usually not considered part of the database, as they are considered an implementation detail, though indices are usually maintained by the same group that maintains the other parts of the database.
Queries made against the relational database, and the derived relvars in the database are expressed in a relational calculus or a relational algebra. Some relvars merely restrict the tuples that are returned, rename the attributes, or remove some of the attributes (often: project) from the result set. These three operations are called unary operations. Other operations that can be performed involve combining more than one relvar together. Examples of these are set union, set intersection, cartesian product (often called a "cross product"), and various different kinds of joins. These are all binary operations.
Debt, AIDS, Trade in Africa (or DATA) is a multinational non-government organization founded in January 2002 in London by U2's Bono along with Bobby Shriver and activists from the Jubilee 2000 Drop
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Ingres (pronounced /iŋ-grεs'/) is a commercially supported, open-source relational database management system.
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In mathematics, a tuple is a finite sequence (also known as an "ordered list") of objects, each of a specified type. A tuple containing n objects is known as an "n-tuple".
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The term relational database was originally defined and coined by E.F. Codd.[1]
Definitions
A relational database is a database that conforms to the relational model, and could also be defined as a set of relations or a database built in an RDBMS.A relational database management system (RDBMS) is a system that manages data using the relational model. Frequently, the term "RDBMS" is inaccurately used as a generic label for the relational database concept. Most current RDBMSs (for example: MySQL, PostgreSQL, Oracle, Microsoft SQL Server, Ingres) deviate significantly from the relational model and are more accurately called SQL database management products. See current usage for an explanation of the requirements for a DBMS to fully support the relational model.
Contents
Strictly, a relational database is a collection of relations (frequently called tables). Other items are frequently considered part of the database, as they help to organize and structure the data, in addition to forcing the database to conform to a set of requirements.Relations or tables
A relation is defined as a set of tuples that all have the same attributes. This is usually represented by a table, which is data organized in rows and columns. In a relational database, all of the data stored in a column should be in the same domain (i.e. data type). In the relational model, the tuples should not have any ordering. This means both that there should be no order to the tuples, and that the tuples should not impose an order of the attributes. Put differently, neither the rows nor the columns should have an order to them.
While this is the desired result, it is not universally achieved. The SQL standard requires columns to have a defined order. All data stored in a computer has to have an order, as the memory of a computer is linear. Also, when the data is returned, there must be an order in which the data is returned (because all transfer protocols are linear, and coincidentally enough, humans read in a linear fashion). The point here is that this order must never make a logical difference in the system. Frequently orders are imposed which impact performance, but they should never change the result of a query on the database. In practice, several of the DBMSs that are considered "relational" impose an order that makes a logical difference.
Constraints
Constraints are a way of providing restrictions on the kinds of data that can be stored in the relations. These are usually defined (formally) in the form of expressions that result in a boolean value, indicating whether or not the constraint holds. Constraints are a way of implementing business rules into the database.
Under the strictest sense, constraints are not considered part of the relational database, but because of the integral role which they play in organizing data, they are usually considered part of the database.
Data domain
A data domain (or usually just domain), is the set of possible values for a given attribute. Because it does constrain the values the data can hold, it could be considered a constraint, but because attributes must specify a domain, it could just be considered part of the relation's definition. Mathematically, a domain can be expressed as "all values for this attribute must be an element of the specified set."
Keys
A tuple usually represents some object and its associated data, whether that object is a physical object or a concept. A key is a kind of constraint which requires that the object, or critical information about the object, isn't duplicated. For example, a family might like to have a constraint such that no two people in the immediate family have the same name. If information about family members were stored in a database, a key could be placed over the family member's name. In a University, they have no such luxury. Each student is typically assigned a Student ID, which are used as keys for individual students stored in the school database. Keys can have more than one column, for example, a nation may impose a restriction that a province can't have two cities by the same name. So, when cities are stored in a relation, there would be a key defined over province and city name. This would allow for two different provinces to have a town called Springfield (because their province would be different), but not two cities with the same name in the same province. A key over more than one attribute is called a compound key. Theoretically, a key can even be over zero attributes. This would enforce that there cannot be more than one tuple in the relation.
Most relations have at least one key defined on it. Because a relation is defined in the relational model as being a set, it can't have duplicate rows. Some DBMSs don't enforce this. If a DBMS does enforce this, it means that there is always at least one key on each relation, namely the key involving all of the attributes of the relation.
A key could be defined formally by requiring that the cardinality of the relation should be equal to the cardinality of the relation projected over the columns of the key.
A key, in this context, refers to any set of attributes which uniquely span the relation. In particular, this is called a superkey. A candidate key is a minimal superkey, meaning that, none of the attributes in the key could be removed from the key, and still have that attribute set be a key. Many DBMSs have a concept of a primary key. The primary key (usually a candidate key) is the key most often used to identify a tuple. In some RDBMSs, the primary key of a base relvar is the storage key (sometimes clustered key), meaning that that is how the data is stored physically. If the value of the primary key is actual interesting data with logical ties to the data (like a name) for the tuple, it is called a natural key. If the key is generated and doesn't have any logical connection to the rest of the data in the tuple, it is called a surrogate key. Other candidate keys that were not chosen as the primary key are called alternate keys.
Foreign keys
A foreign key is not a key by the previous definition. Rather, a foreign key is a reference to a key in another table. Meaning that the referencing tuple has, as part of its attributes, the values of a key in the referenced tuple that corresponds to the relationship.
A foreign key could be described formally as "For all tuples in the referencing relation projected over the referencing attributes, there must exist a tuple in the referenced relation projected over those same attributes such that the values in each of the referencing attributes match the corresponding values in the referenced attributes".
Transition constraints
A transition constraint is a way of enforcing that the data doesn't enter an impossible state because of a previous state. For example, it shouldn't be possible for a person to change from being "married" to being "single, never married". The only valid states after "married" might be "divorced", "widowed", or "deceased".
Other constraints
Other constraints of various different kinds can be created to enforce various kinds of business rules. They can be as simple as "the number of cars an individual owns must be non-negative" or complex patterns like "If the work that an employee performs is 'Hazardous Materials Transport' then that employee's age must be at least 18 years, and the employee's certifications must include 'Hazmat endorsement', and company insurance for that employee must include life insurance."Relvars
A relvar is a "relation variable". In a relational database, all data are stored and accessed via relations. The data that are actually stored in the database are stored as relations. These relations are sometimes called "base relvars". This is equivalent to a "table". Other relvars do not have their data stored in them, but are a result of applying relational operations, to other relvars. These relvars are sometimes called "derived relvars", meaning that their information is derived from other sources. These are equivalent to "views" or "queries". Derived relvars are convenient in that though they may grab information from several relvars, it is presented externally as a single relvar for a simpler perspective. Also, it can be used as an abstraction layer.
Derived relvars are not always considered part of a relational database, partially because they are not essential to the functioning of the database.
Stored procedures
A stored procedure is executable code that is associated with the database. Stored procedures usually store how to perform common operations, like inserting a tuple into a relation, or gathering statistical information about usage patterns. Frequently they are used as an application programming interface (API) for security or simplicity. These are usually written as Imperative programming code extending the Data Definition Language and/or the Data Manipulation Language for the DBMS.
Stored procedures are not always considered part of a relational database, partially because they are not essential to the functioning of the database.
Indices
An index is a way of providing quicker access to the data in a relational database. Indices can be created on any combination of attributes on a relation. Then when tuples in a relation need to be looked up, similar to how a book's index works, the index can be accessed. Rather than having to check all of the tuples, the index tells the DBMS where the tuple is. Indices are usually implemented via B+ trees.
Indices are usually not considered part of the database, as they are considered an implementation detail, though indices are usually maintained by the same group that maintains the other parts of the database.
Relational operations
Queries made against the relational database, and the derived relvars in the database are expressed in a relational calculus or a relational algebra. Some relvars merely restrict the tuples that are returned, rename the attributes, or remove some of the attributes (often: project) from the result set. These three operations are called unary operations. Other operations that can be performed involve combining more than one relvar together. Examples of these are set union, set intersection, cartesian product (often called a "cross product"), and various different kinds of joins. These are all binary operations.
Normalization
References
1. ^ Codd, E.F. (1970). "A Relational Model of Data for Large Shared Data Banks". Communications of the ACM 13 (6): 377-387.
External links
- Database in Depth by Chris Date. ISBN 0-596-10012-4. A new book (2005) by perhaps the best-known author in this field.
- Introduction to Relational Database Design
- Introduction to Relational Databases, 15 Seconds.
- Introduction to Relational Databases, Database Journal.
- Limitations of the relational model
- Relational Database Principles by Colin Ritchie. ISBN 0-8264-5713-4
Topics in database management systems (DBMS)
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|---|---|
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Concepts Database Database models Database storage Relational model Distributed DBMS ACID Null Relational database Relational algebra Relational calculus Database normalization Referential integrity Relational DBMS Primary key, Foreign key, Surrogate key, Superkey, Candidate key | |
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Objects Trigger View Table Cursor Log Transaction Index Stored procedure Partition |
Topics in SQL Select Insert Update Merge Delete Join Union Create Drop Begin work Commit Rollback Truncate Alter |
| Implementations of database management systems | |
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Types of implementations Relational Flat file Deductive Dimensional Hierarchical Object oriented Object relational Temporal XML data stores | |
|
Database products Object-oriented (comparison) Relational (comparison) |
Components Query language Query optimizer Query plan ODBC JDBC |
database is a structured collection of records or data that is stored in a computer system so that a computer program or person using a query language can consult it to answer queries. The records retrieved in answer to queries are information that can be used to make decisions.
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The relational model for database management is a database model based on predicate logic and set theory. It was first formulated and proposed in 1969 by Edgar Codd with aims that included avoiding, without loss of completeness, the need to write computer programs to
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- For other uses, see Data (disambiguation).
Debt, AIDS, Trade in Africa (or DATA) is a multinational non-government organization founded in January 2002 in London by U2's Bono along with Bobby Shriver and activists from the Jubilee 2000 Drop
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A Logical Schema is a data model of a specific problem domain that is in terms of a particular data management technology. Without being specific to a particular database management product, it is in terms of either (for example, in 2007) relational tables and columns,
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A relational database management system (RDBMS) is a database management system (DBMS) that is based on the relational model as introduced by E. F. Codd. Relational databases are the most common kind of database in use today (assuming one does not count a file system as a database).
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Edgar Frank "Ted" Codd
Born July 23 1923
Isle of Portland, England
Died March 18 2003 (aged 81)
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Born July 23 1923
Isle of Portland, England
Died March 18 2003 (aged 81)
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The relational model for database management is a database model based on predicate logic and set theory. It was first formulated and proposed in 1969 by Edgar Codd with aims that included avoiding, without loss of completeness, the need to write computer programs to
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Relation may refer to:
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- Relation, a person to whom one is related, i.e. a family member (see also Kinship)
- Relation (mathematics), a generalization of arithmetic relations, such as "=" and "<", that occur in statements, such as "5
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database is a structured collection of records or data that is stored in a computer system so that a computer program or person using a query language can consult it to answer queries. The records retrieved in answer to queries are information that can be used to make decisions.
..... Click the link for more information.
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A relational database management system (RDBMS) is a database management system (DBMS) that is based on the relational model as introduced by E. F. Codd. Relational databases are the most common kind of database in use today (assuming one does not count a file system as a database).
..... Click the link for more information.
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The relational model for database management is a database model based on predicate logic and set theory. It was first formulated and proposed in 1969 by Edgar Codd with aims that included avoiding, without loss of completeness, the need to write computer programs to
..... Click the link for more information.
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MySQL (pronounced (IPA) /mɑɪ ɛs kjuː ɛl/, "my S-Q-L"[1]) is a multithreaded, multi-user SQL database management system (DBMS)[2]
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PostgreSQL is an object-relational database management system (ORDBMS). It is released under a BSD-style license and is thus free software. As with many other open-source programs, PostgreSQL is not controlled by any single company, but relies on a global community of developers
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Oracle Database (commonly referred to as Oracle RDBMS or simply as Oracle), a relational database management system (RDBMS) software product released by Oracle Corporation, has become a major feature of database computing.
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Microsoft SQL Server is a relational database management system (RDBMS) produced by Microsoft. Its primary query language is Transact-SQL, an implementation of the ANSI/ISO standard Structured Query Language (SQL) used by both Microsoft and Sybase.
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- For the artist, see Jean Auguste Dominique Ingres.
Ingres (pronounced /iŋ-grεs'/) is a commercially supported, open-source relational database management system.
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relation or relationship is a generalization of 2-place relations, such as the relation of equality, denoted by the sign "=" in a statement like "5 + 7 = 12," or the relation of order,
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table is a set of data elements (values) that is organized using a model of horizontal rows and vertical columns. The columns are identified by name, and the rows are identified by the values appearing in a particular column subset which has been identified as a candidate key.
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relation or relationship is a generalization of 2-place relations, such as the relation of equality, denoted by the sign "=" in a statement like "5 + 7 = 12," or the relation of order,
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table is a set of data elements (values) that is organized using a model of horizontal rows and vertical columns. The columns are identified by name, and the rows are identified by the values appearing in a particular column subset which has been identified as a candidate key.
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For the musical term, see .
In mathematics, a tuple is a finite sequence (also known as an "ordered list") of objects, each of a specified type. A tuple containing n objects is known as an "n-tuple".
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The word attribute can refer to:
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- An attribute in philosophy, property, an abstraction of a characteristic of an entity or substance
- An attribute in art, an object that identifies a figure, most commonly referring to objects held by saints - see emblem
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In the context of a relational database, a row—also called a record or tuple—represents a single, implicitly structured data item in a table. In simple terms, a database table can be thought of as consisting of rows and columns or fields.
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In the context of a relational database table, a column is a set of data values of a particular simple type, one for each row of the table.[1] The columns provide the structure according to which the rows are composed.
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domain is most often defined as the set of values, D for which a function is defined.[1] A function that has a domain N is said to be a function over N, where N is an arbitrary set.
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In programming languages a data type defines a set of values and the allowable operations on those values[1]. For example, in the Java programming language, the "int" type represents the set of 32-bit integers ranging in value from -2,147,483,648 to 2,147,483,647, and
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SQL
Paradigm: multi-paradigm
Appeared in: 1974
Designed by: Donald D. Chamberlin and Raymond F. Boyce
Developer: IBM
Latest release: SQL:2003/ 2003
Typing discipline: static, strong
Major implementations: Many
SQL
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Paradigm: multi-paradigm
Appeared in: 1974
Designed by: Donald D. Chamberlin and Raymond F. Boyce
Developer: IBM
Latest release: SQL:2003/ 2003
Typing discipline: static, strong
Major implementations: Many
SQL
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Constraint may refer to:
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- Constraint (mathematics)
- Constraint algorithm (mechanics) such as SHAKE, or LINCS
- Constraint (design)
- Constraint (information theory)
- Theory of Constraints, in business management
- Constraint satisfaction, in computer science
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Boolean may refer to:
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- Boolean datatype, a certain datatype in computer science
- Boolean algebra (logic), a logical calculus of truth values or set membership
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Business rules or business rulesets describe the operations, definitions and constraints that apply to an organization in achieving its goals. For example a business rule might state that no credit check is to be performed on return customers.
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