Concepts inConsistent query answering under primary keys: a characterization of tractable queries

Unique key

In an entity relationship diagram of a data model, one or more unique keys may be declared for each data entity. Each unique key is composed from one or more data attributes of that data entity. The set of unique keys declared for a data entity is often referred to as the candidate keys for that data entity. From the set of candidate keys, a single unique key is selected and declared the primary key for that data entity.
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Boolean conjunctive query

In the theory of relational databases, a Boolean conjunctive query is a conjunctive query without distinguished predicates, i.e. , a query in the form, where each is a relation symbol and each is a tuple of variables and constants; the number of elements in is equal to the arity of . Such a query evaluates to either true or false depending on whether the relations in the database contains the appropriate tuples of values.
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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. Conjunctive queries also have a number of desirable theoretical properties that larger classes of queries (e.g. , the relational algebra queries) do not share.
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Tuple

In mathematics and computer science, a tuple is an ordered list of elements. In set theory, an (ordered) -tuple is a sequence (or ordered list) of elements, where is a positive integer. There is also one 0-tuple, an empty sequence. An -tuple is defined inductively using the construction of an ordered pair. Tuples are usually written by listing the elements within parentheses "" and separated by commas; for example, denotes a 5-tuple.
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First-order logic

First-order logic is a formal system used in mathematics, philosophy, linguistics, and computer science. It is also known as first-order predicate calculus, the lower predicate calculus, quantification theory, and predicate logic (a less precise term). First-order logic is distinguished from propositional logic by its use of quantified variables.
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Directed acyclic graph

In mathematics and computer science, a directed acyclic graph, is a directed graph with no directed cycles. That is, it is formed by a collection of vertices and directed edges, each edge connecting one vertex to another, such that there is no way to start at some vertex v and follow a sequence of edges that eventually loops back to v again. DAGs may be used to model several different kinds of structure in mathematics and computer science.
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If and only if

¿ ¿ ¿ Logical symbolsrepresenting iff In logic and related fields such as mathematics and philosophy, if and only if (shortened iff) is a biconditional logical connective between statements. In that it is biconditional, the connective can be likened to the standard material conditional ("only if," equal to "if ... then") combined with its reverse ("if"); hence the name. The result is that the truth of either one of the connected statements requires the truth of the other, i.e.
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Database

A database is an organized collection of data, today typically in digital form. The data are typically organized to model relevant aspects of reality (for example, the availability of rooms in hotels), in a way that supports processes requiring this information (for example, finding a hotel with vacancies). The term database is correctly applied to the data and their supporting data structures, and not to the database management system (DBMS).
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