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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 simply a key, of a relational database is any set of columns that have a unique combination of values in each row, with the additional constraint that removing any column could produce duplicate combinations of values. A candidate key is a minimal superkey, [1] i.e., a superkey that doesn't contain a smaller one. Therefore ...
The foreign key is typically a primary key of an entity it is related to. The foreign key is an attribute of the identifying (or owner , parent , or dominant ) entity set . Each element in the weak entity set must have a relationship with exactly one element in the owner entity set, [ 1 ] and therefore, the relationship cannot be a many-to-many ...
Codd's twelve rules [1] are a set of thirteen rules (numbered zero to twelve) proposed by Edgar F. Codd, a pioneer of the relational model for databases, designed to define what is required from a database management system in order for it to be considered relational, i.e., a relational database management system (RDBMS).
Fourth normal form (4NF) is a normal form used in database normalization.Introduced by Ronald Fagin in 1977, 4NF is the next level of normalization after Boyce–Codd normal form (BCNF).
There may be several keys in any given table. For example, in a table of employees, both employee number and login name are individually unique. The enforcement of a key constraint (i.e. a uniqueness constraint) in a table is also a data integrity feature of the database. The DBMS prevents updates that would cause duplicate key values and ...
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Reserved words in SQL and related products In SQL:2023 [3] In IBM Db2 13 [4] In Mimer SQL 11.0 [5] In MySQL 8.0 [6] In Oracle Database 23c [7] In PostgreSQL 16 [1] In Microsoft SQL Server 2022 [2]