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Union Of Conjunctive Queries
In database theory, a conjunctive query is a restricted form of first-order queries using the logical conjunction operator. Many first-order queries can be written as conjunctive queries. In particular, a large part of queries issued on relational databases can be expressed in this way. Conjunctive queries also have a number of desirable theoretical properties that larger classes of queries (e.g., the relational algebra queries) do not share. Definition The conjunctive queries are the fragment of (domain independent) first-order logic given by the set of formulae that can be constructed from atomic formulae using conjunction ∧ and existential quantification ∃, but not using disjunction ∨, negation ¬, or universal quantification ∀. Each such formula can be rewritten (efficiently) into an equivalent formula in prenex normal form, thus this form is usually simply assumed. Thus conjunctive queries are of the following general form: :(x_1, \ldots, x_k).\exists x_, \ldot ... [...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] |
Datalog
Datalog is a declarative logic programming language. While it is syntactically a subset of Prolog, Datalog generally uses a bottom-up rather than top-down evaluation model. This difference yields significantly different behavior and properties from Prolog. It is often used as a query language for deductive databases. Datalog has been applied to problems in data integration, networking, program analysis, and more. Example A Datalog program consists of ''facts'', which are statements that are held to be true, and ''rules'', which say how to deduce new facts from known facts. For example, here are two facts that mean ''xerces is a parent of brooke'' and ''brooke is a parent of damocles'': parent(xerces, brooke). parent(brooke, damocles). The names are written in lowercase because strings beginning with an uppercase letter stand for variables. Here are two rules: ancestor(X, Y) :- parent(X, Y). ancestor(X, Y) :- parent(X, Z), ancestor(Z, Y). The :- symbol is read as "if", ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Ashok K
Ashoka (died 232 BC) was a monarch of the Mauryan Empire of India. Ashoka, Asoka, or Ashok may also refer to: Entertainment * ''Ashok'' (film), a 2006 Telugu film directed by Surender Reddy * ''Ashoka'' (2006 film), a 2006 Kannada film directed by Shivamani * ''Ashoka'' (2008 film), a 2008 Tamil film directed by Prem * ''Asoka'' (1955 film), a 1955 Sri Lankan film directed by Sirisena Wimalaweera * ''Aśoka'' (film), a 2001 Bollywood historical drama film directed by Santosh Sivan * ''Ashoka the Great'' (book), a 2011 book by Wytze Keuning * '' Ashoka the Hero'', a 2011 Indian animated film * Ashok, a fictional character portrayed by Danny Denzongpa in his 1980 Indian film '' Phir Wohi Raat'' People Actors * Ashok Kumar (Tamil actor) (born 1981), South Indian actor and Bharatnatyam performer * Ashok Kumar (1911–2001), Indian film actor * Ashok Lokhande (born 1950), Indian film actor * Ashok Mehta (1947–2012), Indian film actor * Ashok Saraf (born 1947), Indian actor ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Combined Complexity
In database theory, the query evaluation problem is the problem of determining the answers to a query on a database. Research in database theory aims at determining the computational complexity of answering different kinds of queries over databases, in particular over relational databases. Formal definition The query evaluation problem takes two inputs: the query to be answered, and the database on which to answer it. The output of the problem is the set of answers to the query on the database. If the queries are ''Boolean queries'', i.e., queries have a yes or no answer (for example, Boolean conjunctive queries) then the query evaluation problem is a decision problem. The query evaluation problem is usually posed for a specific class of queries and databases. For instance, one example of the query evaluation problem would be the problem of evaluating a conjunctive query on a relational database. The computational complexity of the problem can be measured in different ways ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
NP-complete
In computational complexity theory, NP-complete problems are the hardest of the problems to which ''solutions'' can be verified ''quickly''. Somewhat more precisely, a problem is NP-complete when: # It is a decision problem, meaning that for any input to the problem, the output is either "yes" or "no". # When the answer is "yes", this can be demonstrated through the existence of a short (polynomial length) ''solution''. # The correctness of each solution can be verified quickly (namely, in polynomial time) and a brute-force search algorithm can find a solution by trying all possible solutions. # The problem can be used to simulate every other problem for which we can verify quickly that a solution is correct. Hence, if we could find solutions of some NP-complete problem quickly, we could quickly find the solutions of every other problem to which a given solution can be easily verified. The name "NP-complete" is short for "nondeterministic polynomial-time complete". In this name, ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Analysis Of Algorithms
In computer science, the analysis of algorithms is the process of finding the computational complexity of algorithms—the amount of time, storage, or other resources needed to execute them. Usually, this involves determining a function that relates the size of an algorithm's input to the number of steps it takes (its time complexity) or the number of storage locations it uses (its space complexity). An algorithm is said to be efficient when this function's values are small, or grow slowly compared to a growth in the size of the input. Different inputs of the same size may cause the algorithm to have different behavior, so best, worst and average case descriptions might all be of practical interest. When not otherwise specified, the function describing the performance of an algorithm is usually an upper bound, determined from the worst case inputs to the algorithm. The term "analysis of algorithms" was coined by Donald Knuth. Algorithm analysis is an important part of a broa ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Relational Databases
A relational database (RDB) is a database based on the relational model of data, as proposed by E. F. Codd in 1970. A Relational Database Management System (RDBMS) is a type of database management system that stores data in a structured format using rows and columns. Many relational database systems are equipped with the option of using SQL (Structured Query Language) for querying and updating the database. History The concept of relational database was defined by E. F. Codd at IBM in 1970. Codd introduced the term ''relational'' in his research paper "A Relational Model of Data for Large Shared Data Banks". In this paper and later papers, he defined what he meant by ''relation''. One well-known definition of what constitutes a relational database system is composed of Codd's 12 rules. However, no commercial implementations of the relational model conform to all of Codd's rules, so the term has gradually come to describe a broader class of database systems, which at a m ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Aggregate Function
In database management, an aggregate function or aggregation function is a function where multiple values are processed together to form a single summary statistic. Common aggregate functions include: * Average (i.e., arithmetic mean) * Count * Maximum * Median * Minimum * Mode * Range * Sum Others include: * Nanmean (mean ignoring NaN values, also known as "nil" or "null") * Stddev Formally, an aggregate function takes as input a set, a multiset (bag), or a list from some input domain and outputs an element of an output domain . The input and output domains may be the same, such as for SUM, or may be different, such as for COUNT. Aggregate functions occur commonly in numerous programming languages, in spreadsheets, and in relational algebra. The listagg function, as defined in the SQL:2016 standard aggregates data from multiple rows into a single concatenated string. In the entity relationship diagram, aggregation is represented as seen in Figure 1 with a rectang ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Expressive Power (computer Science)
In computer science, the expressive power (also called expressiveness or expressivity) of a language is the breadth of ideas that can be represented and communicated in that language. The more expressive a language is, the greater the variety and quantity of ideas it can be used to represent. For example, the Web Ontology Language expression language profile (OWL2 EL) lacks ideas (such as negation) that can be expressed in OWL2 RL (rule language). OWL2 EL may therefore be said to have less ''expressive power'' than OWL2 RL. These restrictions allow for more efficient ( polynomial time) reasoning in OWL2 EL than in OWL2 RL. So OWL2 EL trades some expressive power for more efficient reasoning (processing of the knowledge representation language). Information description The term ''expressive power'' may be used with a range of meaning. It may mean a measure of the ideas expressible in that language: * regardless of ease (''theoretical expressivity'') * concisely and readily ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Moshe Y
Moshe is the Hebrew version of the masculine given name Moses. Bearers include: * Moshe Arens (1925–2019), Israeli politician * Moshe Bar, several people * Moshe Bejski (1921–2007), Israeli judge * Moshe Brener (born 1971), Israeli basketball player * Moshe Czerniak (1910–1984), Israeli chess master * Moshe Dayan (1915–1981), Israeli military leader and politician * Moshe Erem (1896–1978), Israeli politician * Moshe Feinstein (1895–1986), Russian-born American Orthodox Jewish rabbi, scholar, and posek * Moshe Gil (1921–2014), Israeli historian * Moshe Gutnick, Australian Orthodox Chabad rabbi * Moshe Hirsch (1929–2010), Jewish activist and Palestinian politician * Moshe Ivgy (born 1953), Israeli actor * Moshe Jarden (born 1942), Israeli mathematician * Moshe Kahlon (born 1960) Israeli politician * Moshe Kasher (born 1979), American comedian * Moshe Katsav (born 1945), Israeli-Iranian president of Israel * Moshe Katz, several people * Moshe Kaveh (born 1943 ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Harry G
Harry may refer to: Television * ''Harry'' (American TV series), 1987 comedy series starring Alan Arkin * ''Harry'' (British TV series), 1993 BBC drama that ran for two seasons * ''Harry'' (New Zealand TV series), 2013 crime drama starring Oscar Kightley * ''Harry'' (talk show), 2016 American daytime talk show hosted by Harry Connick Jr. People and fictional characters *Harry (given name), a list of people and fictional characters with the given name, including **Prince Harry, Duke of Sussex (born 1984) *Harry (surname), a list of people with the surname Other uses *"Harry", the tunnel used in the Stalag Luft III escape ("The Great Escape") of World War II * ''Harry'' (album), a 1969 album by Harry Nilsson *Harry (derogatory term) Harry is a Norwegian derogatory term used in slang, derived from the English name Harry. The best English translation may be "cheesy" or "tacky". '' Norsk ordbok'' defines "harry" as "tasteless, vulgar". The term "harry" was first used by upper ... ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Paris C
Paris () is the Capital city, capital and List of communes in France with over 20,000 inhabitants, largest city of France. With an estimated population of 2,048,472 residents in January 2025 in an area of more than , Paris is the List of cities in the European Union by population within city limits, fourth-most populous city in the European Union and the List of cities proper by population density, 30th most densely populated city in the world in 2022. Since the 17th century, Paris has been one of the world's major centres of finance, diplomacy, commerce, culture, Fashion capital, fashion, and gastronomy. Because of its leading role in the French art, arts and Science and technology in France, sciences and its early adoption of extensive street lighting, Paris became known as the City of Light in the 19th century. The City of Paris is the centre of the Île-de-France region, or Paris Region, with an official estimated population of 12,271,794 inhabitants in January 2023, or ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
