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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] |
Database Schema
The database schema is the structure of a database described in a formal language supported typically by a relational database management system (RDBMS). The term "wikt:schema, schema" refers to the organization of data as a blueprint of how the database is constructed (divided into database tables in the case of relational databases). The formal definition of a database schema is a set of formulas (sentences) called integrity constraints imposed on a database. These integrity constraints ensure compatibility between parts of the schema. All constraints are expressible in the same language. A database can be considered a structure in realization of the database language. The states of a created conceptual schema are transformed into an explicit mapping, the database schema. This describes how real-world entities are Data modeling, modeled in the database. "A database schema specifies, based on the database administrator's knowledge of possible applications, the facts that can ent ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Boyce–Codd Normal Form
Boyce–Codd normal form (BCNF or 3.5NF) is a normal form used in database normalization. It is a slightly stricter version of the third normal form (3NF). By using BCNF, a database will remove all redundancies based on functional dependencies. History Edgar F. Codd released his original article "A Relational Model of Data for Large Shared Databanks" in June 1970. This was the first time the notion of a relational database was published. All work after this, including the Boyce–Codd normal form method was based on this relational model. The Boyce–Codd normal form was first described by Ian Heath in 1971, and has also been called Heath normal form by Chris Date. BCNF was formally developed in 1974 by Raymond F. Boyce and Edgar F. Codd to address certain types of anomalies not dealt with by 3NF as originally defined.Codd, E. F. "Recent Investigations into Relational Data Base" in ''Proc. 1974 Congress'' (Stockholm, Sweden, 1974). New York, N.Y.: North-Holland (1974). As ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Relational Projection
In relational algebra, a projection is a unary operation written as \Pi_( R ), where R is a relation and a_1,...,a_n are attribute names. Its result is defined as the set obtained when the components of the tuples in R are restricted to the set \ – it ''discards'' (or ''excludes'') the other attributes. In practical terms, if a relation is thought of as a table, then projection can be thought of as picking a subset of its columns. For example, if the attributes are (name, age), then projection of the relation onto attribute list (age) yields – we have discarded the names, and only know what ages are present. Projections may also modify attribute values. For example, if R has attributes a, b, c, where the values of b are numbers, then \Pi_( R ) is like R, but with all b-values halved.http://www.csee.umbc.edu/~pmundur/courses/CMSC661-02/rel-alg.pdf ''See Problem 3.8.B on page 3'' Related concepts The closely related concept in set theory (see: projection (set theory)) differs ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Composite Key
In database design, a composite key is a candidate key that consists of two or more attributes, (table columns) that together uniquely identify an entity occurrence (table row). A compound key is a composite key for which each attribute that makes up the key is a foreign key in its own right. Advantages Composite keys have advantages similar to that of a natural key as it is often composed of multiple natural key attributes. Storage Composite keys use less disk space as compared to defining a surrogate key column, this is because the composite key already exists as attributes in the table and does not need to be defined in the table just for the purpose of unique identification. This simplifies the table and also saves space. Easier to implement and use Composite keys are easy to implement in a database schema as their component parts are already named items in the database. When they are also natural keys, they are often intuitive for real world scenarios. They are ofte ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Christopher J
Christopher is the English version of a Europe-wide name derived from the Greek name Χριστόφορος (''Christophoros'' or '' Christoforos''). The constituent parts are Χριστός (''Christós''), "Christ" or "Anointed", and φέρειν (''phérein''), "to bear"; hence the "Christ-bearer". As a given name, 'Christopher' has been in use since the 10th century. In English, Christopher may be abbreviated as " Chris", "Topher", and sometimes " Kit". It was frequently the most popular male first name in the United Kingdom, having been in the top twenty in England and Wales from the 1940s until 1995, although it has since dropped out of the top 100. Within the United Kingdom, the name is most common in England and not so common in Wales, Scotland, or Northern Ireland. Cognates in other languages *Afrikaans: Christoffel, Christoforus * Albanian: Kristofer, Kristofor, Kristoforid, Kristo *Arabic: كريستوفر (''Krīstafor, Kristūfar, Krístufer''), اصطفر (''ʔ ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Necessity And Sufficiency
In logic and mathematics, necessity and sufficiency are terms used to describe a material conditional, conditional or implicational relationship between two Statement (logic), statements. For example, in the Conditional sentence, conditional statement: "If then ", is necessary for , because the Truth value, truth of is guaranteed by the truth of . (Equivalently, it is impossible to have without , or the falsity of ensures the falsity of .) Similarly, is sufficient for , because being true always implies that is true, but not being true does not always imply that is not true. In general, a necessary condition is one (possibly one of several conditions) that must be present in order for another condition to occur, while a sufficient condition is one that produces the said condition. The assertion that a statement is a "necessary ''and'' sufficient" condition of another means that the former statement is true if and only if the latter is true. That is, the two statements mu ... [...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] |
Sworn Testimony
Sworn testimony is evidence given by a witness who has made a commitment to tell the truth. If the witness is later found to have lied whilst bound by the commitment, they can often be charged with the crime of perjury. The types of commitment can include oaths, affirmations and promises which are explained in more detail below. The exact wording of the commitments vary from country to country. Types of commitment The commitment can come in different forms depending on the situation of the witness. The types of commitment used in the United Kingdom are: *Oath: A commitment made to the witness's deity, or on their holy book. *Affirmation: A secular variant of the oath where the witness does not have to mention a deity or holy book. *Promise: A commitment made by a witness under the age of 17, or of all witnesses if none of the accused are over the age of 17. Variations by country Canada ''Affirmation'': :I solemnly affirm that the evidence to be given by me shall be the truth, th ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Complement (set Theory)
In set theory, the complement of a Set (mathematics), set , often denoted by A^c (or ), is the set of Element (mathematics), elements not in . When all elements in the Universe (set theory), universe, i.e. all elements under consideration, are considered to be Element (mathematics), members of a given set , the absolute complement of is the set of elements in that are not in . The relative complement of with respect to a set , also termed the set difference of and , written B \setminus A, is the set of elements in that are not in . Absolute complement Definition If is a set, then the absolute complement of (or simply the complement of ) is the set of elements not in (within a larger set that is implicitly defined). In other words, let be a set that contains all the elements under study; if there is no need to mention , either because it has been previously specified, or it is obvious and unique, then the absolute complement of is the relative complement of in : ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Superkey
In the relational data model a superkey is any set of attributes that uniquely identifies each tuple of a relation. Because superkey values are unique, tuples with the same superkey value must also have the same non-key attribute values. That is, non-key attributes are functionally dependent on the superkey. The set of all attributes is always a superkey (the trivial superkey). Tuples in a relation are by definition unique, with duplicates removed after each operation, so the set of all attributes is always uniquely valued for every tuple. A candidate key (or minimal superkey) is a superkey that can't be reduced to a simpler superkey by removing an attribute. For example, in an employee schema with attributes employeeID, name, job, and departmentID, if employeeID values are unique then employeeID combined with any or all of the other attributes can uniquely identify tuples in the table. Each combination, , , , and so on is a superkey. is a candidate key, since no subset of ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Henry F
Henry may refer to: People and fictional characters * Henry (given name), including lists of people and fictional characters * Henry (surname) * Henry, a stage name of François-Louis Henry (1786–1855), French baritone Arts and entertainment * ''Henry'' (2011 film), a Canadian short film * ''Henry'' (2015 film), a virtual reality film * '' Henry: Portrait of a Serial Killer'', a 1986 American crime film * ''Henry'' (comics), an American comic strip created in 1932 by Carl Anderson * "Henry", a song by New Riders of the Purple Sage Places Antarctica * Henry Bay, Wilkes Land Australia * Henry River (New South Wales) * Henry River (Western Australia) Canada * Henry Lake (Vancouver Island), British Columbia * Henry Lake (Halifax County), Nova Scotia * Henry Lake (District of Chester), Nova Scotia New Zealand * Lake Henry (New Zealand) * Henry River (New Zealand) United States * Henry, Illinois * Henry, Indiana * Henry, Nebraska * Henry, South Dakota * Henry County ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Abraham Silberschatz
Avi Silberschatz (; born in Haifa, Israel) is an Israeli computer scientist and researcher. He is known for having authored many influential texts in computer science. He finished high school at the Hebrew Reali School in Haifa and graduated in 1976 with a Ph.D. in computer science from the State University of New York (SUNY) at Stony Brook. His research interests include database systems, operating systems, storage systems, and network management. He held a professorship at the University of Texas at Austin, where he taught until 1993. He became a professor at Yale University in 2005, where he was the chair of the Computer Science department from 2005 to 2011. Prior to coming to Yale in 2003, Silberschatz worked at the Bell Labs. Awards and recognition Silberschatz was elected an ACM Fellow in 1996 and received the Karl V. Karlstrom Outstanding Educator Award in 1998. He was elected an IEEE The Institute of Electrical and Electronics Engineers (IEEE) is an Ameri ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
