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Given a set of functional dependencies , an Armstrong relation is a relation which satisfies all the functional dependencies in the closure + and only those dependencies. . Unfortunately, the minimum-size Armstrong relation for a given set of dependencies can have a size which is an exponential function of the number of attributes in the dependencies conside
Every non-trivial functional dependency begins with a superkey (a stricter form of 3NF) — Every non-trivial multivalued dependency begins with a superkey — Every join dependency has a superkey component [8] — Every join dependency has only superkey components — Every constraint is a consequence of domain constraints and key constraints
A data dependency in computer science is a situation in which a program statement (instruction) refers to the data of a preceding statement. In compiler theory , the technique used to discover data dependencies among statements (or instructions) is called dependence analysis .
A database abstraction layer (DBAL [1] or DAL) is an application programming interface which unifies the communication between a computer application and databases such as SQL Server, IBM Db2, MySQL, PostgreSQL, Oracle or SQLite. Traditionally, all database vendors provide their own interface that is tailored to their products.
In a SQL database query, a correlated subquery (also known as a synchronized subquery) is a subquery (a query nested inside another query) that uses values from the outer query. This can have major impact on performance because the correlated subquery might get recomputed every time for each row of the outer query is processed.
Embedded dependencies can express functional dependencies, join dependencies, multivalued dependencies, inclusion dependencies, foreign key dependencies, and many more besides. An algorithm known as the chase takes as input an instance that may or may not satisfy a set of EDs, and, if it terminates (which is a priori undecidable), output an ...
There is a functional dependency from the candidate key to all the attributes in the relation. The superkeys of a relation are all the possible ways we can identify a row. The candidate keys are the minimal subsets of each superkey and as such, they are an important concept for the design of database schema .
Also, self-references are possible (not fully implemented in MS SQL Server though [5]). On inserting a new row into the referencing table, the relational database management system (RDBMS) checks if the entered key value exists in the referenced table. If not, no insert is possible.