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For example, {Monarch Name, Royal House} cannot be a superkey because for the same attribute values (Edward, Plantagenet), there are two distinct tuples: (Edward, II, Plantagenet) (Edward, III, Plantagenet) Finally, after elimination, the remaining sets of attributes are the only possible superkeys in this example:
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 ...
Some possible examples of keys are Social Security Numbers, ISBNs, vehicle registration numbers or user login names. In principle any key may be referenced by foreign keys. Some SQL DBMSs only allow a foreign key constraint against a primary key but most systems will allow a foreign key constraint to reference any key of a table.
This is an important element of SQL. Statements, which may have a persistent effect on schemata and data, or may control transactions, program flow, connections, sessions, or diagnostics. SQL statements also include the semicolon (";") statement terminator. Though not required on every platform, it is defined as a standard part of the SQL grammar.
For example if there is a table storing the information about US citizens, the Social Security Number would act as the natural key, Social Security Number being the natural key might pose a problem in the future if the US government changes the structure of the Social Security Number and increases the number of digits in the SSN due to some reason.
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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 ...
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