What is foreign key, candidate key and alternate key?
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Answer:
A FOREIGN KEY is a key used to link two tables together. A FOREIGN KEY is a field (or collection of fields) in one table that refers to the PRIMARY KEY in another table. The table containing the foreign key is called the child table, and the table containing the candidate key is called the referenced or parent table.
In the relational model of databases, a candidate key of a relation is a minimal superkey for that relation; that is, a set of attributes such that: the relation does not have two distinct tuples with the same values for these attributes there is no proper subset of these attributes for which holds.
An alternate key is a key associated with one or more columns whose values uniquely identify every row in the table, but which is not the primary key. For example, where the primary key for a table may be the employee id, the alternate key might combine the first, middle, and last names of the employee.
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What is the Alternate key?
All the keys which are not primary key are called an alternate key. It is a candidate key which is currently not the primary key. However, A table may have single or multiple choices for the primary key.
What is a Candidate Key?
A super key with no repeated attribute is called candidate key.
The Primary key should be selected from the candidate keys. Every table must have at least a single candidate key.
What is the Foreign key?
A foreign key is a column which is added to create a relationship with another table. Foreign keys help us to maintain data integrity and also allows navigation between two different instances of an entity. Every relationship in the model needs to be supported by a foreign key.