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In the relational model of databases, a primary key is a designated attribute that can reliably identify and distinguish between each individual record in a table.The database creator can choose an existing unique attribute or combination of attributes from the table (a natural key) to act as its primary key, or create a new attribute containing a unique ID that exists solely for this purpose ...
An index may be declared as UNIQUE, which creates an implicit constraint on the underlying table. Database systems usually implicitly create an index on a set of columns declared PRIMARY KEY, and some are capable of using an already-existing index to police this constraint.
A key that may become the primary key. Primary The key that is selected as the primary key. Only one key within an entity is selected to be the primary key. This is the key that is allowed to migrate to other entities to define the relationships that exist among the entities.
Every relation/table has a primary key, this being a consequence of a relation being a set. [21] A primary key uniquely specifies a tuple within a table. While natural attributes (attributes used to describe the data being entered) are sometimes good primary keys, surrogate keys are often used instead. A surrogate key is an artificial attribute ...
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 ...
A hash table uses a hash function to compute an index, also called a hash code, into an array of buckets or slots, from which the desired value can be found. During lookup, the key is hashed and the resulting hash indicates where the corresponding value is stored. A map implemented by a hash table is called a hash map.
Portions of this article were drafted using an in-house natural language generation platform.The article was reviewed, fact-checked and edited by our editorial staff.. Index funds and exchange ...
Since a current database stores only currently valid data, there is a one-to-one correspondence between a surrogate in the modeled world and the primary key of the database. In this case the surrogate may be used as a primary key, resulting in the term surrogate key. In a temporal database, however, there is a many-to-one relationship between ...