Candidate Key in DBMS

In the field of database management systems (DBMS), a candidate key is a set of one or more columns that can uniquely identify a row within a table. A candidate key is also known as a "unique identifier" or "primary key." In this article, we will discuss the importance of candidate keys in DBMS and how they are used to maintain the integrity and consistency of a database.

What is a Candidate Key?

A candidate key is a set of one or more columns that can be used to uniquely identify a row within a table. In other words, no two rows within a table can have the same values for the columns that make up a candidate key. In practice, a table can have multiple candidate keys, but only one of them can be chosen as the primary key. The primary key is used as the official unique identifier for a row within the table.

For example, let's consider a table called "Employees" that contains information about the employees of a company. The table has the following columns: EmployeeID, FirstName, LastName, and Email. In this case, the column "EmployeeID" could be a candidate key because it is unique for each employee and can be used to identify a specific row within the table.

Creating a Candidate Key

Example

Creating a candidate key is a simple process. In most DBMS, you can create a candidate key by defining a primary key constraint on one or more columns of a table. The syntax for creating a primary key constraint varies depending on the DBMS you are using, but in most cases, it looks something like this −

CREATE TABLE Employees (
   EmployeeID INT NOT NULL PRIMARY KEY,
   FirstName VARCHAR(255) NOT NULL,
   LastName VARCHAR(255) NOT NULL,
   Email VARCHAR(255) NOT NULL
);

In the example above, the primary key constraint is placed on the "EmployeeID" column. This means that the "EmployeeID" column is a candidate key for the "Employees" table, and no two rows can have the same value for this column.

Types of Candidate Keys

Candidate keys can be of multiple types, they can be simple or composite, and can be single or multiple.

• A Simple Candidate key is a primary key that is made up of a single column, such as the "EmployeeID" example above.

• A Composite Candidate key is a primary key that is made up of multiple columns. For example, a table of "Orders" could have a composite primary key made up of the columns "OrderID" and "CustomerID". Together, these columns uniquely identify a specific order placed by a customer.

• A Super key is a set of one or more columns that can identify a row uniquely, but not necessarily minimal. It can be either simple or composite.

• A Minimal Super key is a superkey with the least number of attributes.

Why are Candidate Keys Important?

Candidate keys play a vital role in maintaining the integrity and consistency of a database. They ensure that each row within a table is unique and can be identified independently. This allows for accurate data retrieval and helps prevent data duplication and errors.

Candidate keys also help to ensure data consistency by enforcing relationships between tables. For example, in a database with multiple tables, a foreign key can be used to reference the primary key of another table. This creates a relationship between the two tables and ensures that the data is consistent and accurate.

For example, in our "Employees" table, if there is a "Department" table with a primary key of "DepartmentID", a foreign key can be created in the "Employees" table referencing the "DepartmentID" column, linking each employee to the department they belong to.

In addition, candidate keys also help to improve the performance of a database by allowing for faster and more efficient data retrieval. This is because the DBMS can use the primary key to quickly and easily locate specific rows within a table, without having to search through the entire table. This can be especially important for large databases where performance is a critical concern.

Secondary Keys and Non-Key Attributes

In addition to candidate keys, a table may also have secondary keys, which are also unique but are not chosen to be the primary key. Secondary keys can also be used to improve data retrieval and provide an alternative means of identifying a row.

Non-key attributes, on the other hand, are columns within a table that do not form part of any key and do not provide any unique identification for a row. They provide additional information about the entity represented by the row, such as the "FirstName" and "LastName" columns in our "Employees" table example.

Candidate Key vs Primary Key

It's important to note that while the terms "candidate key" and "primary key" are often used interchangeably, they are not the same thing. A candidate key is a set of one or more columns that can uniquely identify a row within a table, while a primary key is a candidate key that is chosen to be the official unique identifier for a row within the table. A table can have multiple candidate keys, but can only have one primary key.

Surrogate Key

A surrogate key is a type of primary key that is used specifically as a unique identifier for a row, rather than using a natural key. A natural key is a column or set of columns that have inherent meaning within the data, such as "EmployeeID" in our "Employees" table example. A surrogate key, on the other hand, is a unique identifier that is generated by the database and has no inherent meaning within the data.

The main advantage of using a surrogate key is that it is generally smaller and more efficient to use as a primary key, and it can also be changed without affecting the data, which may not be possible with a natural key.

One example of a surrogate key can be Auto-Incrementing Integer, These are generally used as primary key, it starts with a number and increase by a fixed number, and it is guaranteed to be unique. This is because the database system assigns the key and keeps track of the last assigned value.

Choosing the Right Candidate Key

It is crucial to choose the right candidate key while designing a database. The choice of candidate key can have a significant impact on the performance, scalability and flexibility of the database. It is important to consider the following factors while choosing a candidate key −

• Uniqueness − The key should be unique and not subject to change.

• Minimum size − The key should be as small as possible to improve performance.

• Stability − The key should not change over time.

• Business meaning − The key should have business meaning and be related to the data in the table.

By considering these factors, it will be easier to choose the right candidate key that will meet the needs of the database, and ensure its proper functioning in the long term.

All of these points provide insights about the types of primary keys and their usage, so you can choose the best candidate key for your database, by keeping in mind the specific requirements of your system and the business.

Conclusion

Candidate keys play a crucial role in maintaining the integrity and consistency of a database. They are used to ensure that each row within a table is unique and can be identified independently, and also used to enforce relationships between tables.

In addition, they can also improve the performance of a database by allowing for faster and more efficient data retrieval. Understanding the types of candidate keys and how to use them effectively is an essential part of managing a DBMS efficiently. It is important to note that having an appropriate candidate key is the first step to designing a good database.