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A candidate key is a minimal superkey, [1] i.e., a superkey that does not contain a smaller one. Therefore, a relation can have multiple candidate keys, each with a different number of attributes. [2] Specific candidate keys are sometimes called primary keys, secondary keys or alternate keys.
In a relational database, a candidate key uniquely identifies each row of data values in a database table. A candidate key comprises a single column or a set of columns in a single database table. No two distinct rows or data records in a database table can have the same data value (or combination of data values) in those candidate key columns ...
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. [4] In database relational modeling and implementation, a candidate key is a set of zero or more attributes, the values of which are guaranteed to be unique for each tuple (row) in a relation. The ...
It does not have any non-prime attribute that is functionally dependent on any proper subset of any candidate key of the relation (i.e. it lacks partial dependencies). A non-prime attribute of a relation is an attribute that is not a part of any candidate key of the relation. Put simply, a relation (or table) is in 2NF if:
Recall that the Book table below has a composite key of {Title, Format}, which will not satisfy 2NF if some subset of that key is a determinant. At this point in our design the key is not finalized as the primary key, so it is called a candidate key. Consider the following table:
In database design, a composite key is a candidate key that consists of two or more attributes, [1] [2] [3] (table columns) that together uniquely identify an entity occurrence (table row). A compound key is a composite key for which each attribute that makes up the key is a foreign key in its own right.
A database relation (e.g. a database table) is said to meet third normal form standards if all the attributes (e.g. database columns) are functionally dependent on solely a key, except the case of functional dependency whose right hand side is a prime attribute (an attribute which is strictly included into some key).
The candidate keys of the table are: {Person, Shop type}, {Person, Nearest shop}. Because all three attributes are prime attributes (i.e. belong to candidate keys), the table is in 3NF. The table is not in BCNF, however, as the Shop type attribute is functionally dependent on a non-superkey: Nearest shop.