Candidate Key


We call the candidate key of a relation (or simply key) to the attribute or set of attributes that have the property of uniquely identifying a tuple within the relation. The keys are the basic tuple level addressing mechanism in a relational system, that is, it is the only way, guaranteed by the system, to locate a specific tuple. Let’s see this in more detail below, so you can know the candidate key definition and create your candidate key in dbms.

What is Candate Key?candidate key image

A row can be differentiated from the others through a number of attributes. These attributes form what is called a candidate key. There may be several candidate keys within a table.

The main or primary key is the candidate key that we choose to represent each row in a unique way. Then it will allow us to identify and locate a record in a fast and organized way.

Determining the candidate keys

To identify the candidate keys of a table do not have to look at a state or instance of the database: that at any given time there are no duplicates for an attribute or set of attributes, it does not guarantee that duplicates are not possible. However, the reciprocal is valid: the presence of duplicates in a state of the base shows that a certain combination of attributes is not a valid candidate key.

An office relationship has an attribute or population, that attribute is not a candidate key since there may be several offices in the same population. As the company assigns a code to each office, the attribute Office N° is a candidate key.

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Function of candidate Key in DBMS

The keys serve two roles in relational databases: relate the data in tables and differentiate the records from one to the other. Primary keys are one or more fields used to link the tables. For example, the primary key of the “Customer” table “CustomerID” is the foreign key in the “Order” table that links the order to a specific customer.

Characteristics of Candidate Key

Each table can have several keys of the candidate; for example, the “Customer” table can have two candidate keys. “CustomerId” as the only candidate key field and the combination of “Name”, “Last Name” and “Email” may be different. One of the candidate keys is selected as the primary key of the table. In general, the best option uses a single field, such as the “ID” field, to optimize the performance of queries.

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