Last Updated on April 28, 2023 by Prepbytes
Candidate Key in DBMS is an essential concept in database design and is used to ensure the accuracy and consistency of data stored in a database. Here, we will discuss the candidate key, key in DBMS, candidate key in dbms, super key, the difference between super key vs candidate key, how to find candidate key in dbms, and some examples of candidate key in dbms. Whether you are a beginner or an experienced database developer, understanding candidate keys is crucial for creating effective and robust database systems.
Key in DBMS
Keys in DBMS (Database Management System) are used to uniquely identify records or rows in a table. Different types of keys exist in DBMS, such as primary keys, foreign keys, and candidate keys. Each key serves a specific purpose in ensuring data accuracy and consistency in a database system as given below.
- A primary key is a unique identifier for each record, and cannot be null.
- A foreign key is a key that is used to link two tables together.
- A candidate key is a set of one or more fields that can be used as a primary key but are not necessarily used as such.
What is Candidate Key in DBMS?
Candidate Key in DBMS is a subset of a super key or a set of one or more attributes that can uniquely identify a tuple (row) in a relation (table) in a relational database. A relation in a database can possess multiple candidate keys, although it must have at least one. Among these candidate keys, one is selected to uniquely identify the records in the relation and is referred to as the primary key.
Define Candidate Key in DBMS
In a database management system (DBMS), a candidate key is a set of one or more attributes within a table that can uniquely identify each record or row in that table. This means that no two records can have the same values for all the attributes in the candidate key. A table can have multiple candidate keys, but at least one of them must be selected as the primary key to uniquely identify each record in the table. Candidate keys are also used to create indexes, which are used to speed up the retrieval of data from the database.
Example of Candidate Key in DBMS
Let us understand the concept of candidate key in DBMS with the help of the following Student table.
Student_Rollno,Student_Name, Student_ID_no, and Student_email. These are the attributes of the student_detail table, in which some are unique and some are not. here Student_Name is not unique because the name of the student can be the same as we observe Student_Rollno 01 and 04 have the same name (Vikas) so this column is not unique. So Student_Name cannot use to find unique tuple individuality so as we observe in the super key table (Student_Name+Student_Rollno, Student_email) and a candidate is part of the super key or subset of superkey.
| Student_Rollno | Student_Name | Student_ID_no | Student_email |
|---|---|---|---|
| 01 | Vikas | 1233329 | v@gmail.com |
| 02 | Mohan | 1234534 | m@gmail.com |
| 03 | Lalit | 1234328 | l@gmail.com |
| 04 | Vikas | 1239875 | vk@gmail.com |
Set of Super Keys Obtained
{ Student_Rollno }
{Student_ID_no}
{Student_email}
{Student_Rollno, Student_ID_no}
{ Student_Rollno, Student_email}
{Student_Rollno, Student_email, Student_Name}
{Student_Rollno, Student_email, Student_ID_no}Candidate Keys
Student_Rollno
Student_ID_no
Student_emailAdvantages of Candidate Key in DBMS
The main advantages of candidate keys in DBMS are given below.
- Uniqueness: The candidate key in DBMS uniquely identifies the relation and ensures that no two tuples are identical.
- Minimization of data redundancy: Since each tuple can be uniquely identified by its candidate key, there is no need to store duplicate information in the relation.
- Improved data integrity: By utilizing candidate keys in DBMS, the principle of "entity integrity" is enforced. This principle mandates that every tuple within a relation has a distinct identifier, which ensures the reliability and consistency of the data in the relation.
- Improved data organization: Candidate keys can be used to organize data into logical groups, making it easier to retrieve and manipulate the data.
- Simplification of database design: The use of candidate keys can simplify the design of database relationships, as it is clear which attributes are used to link tables together.
What is Super Key?
A super key in DBMS is a collection of one or more attributes, or columns, that can uniquely identify a row, or tuple, within a table. In other words, a super key is a set of attributes that can be used to uniquely identify a specific row within a table. A candidate key is a minimal super key, meaning it is a super key with the least number of attributes. A primary key is a candidate key that is selected to be the main identifier for the table.
Candidate Key vs Super Key
A candidate key is a minimal super key in a relational database table. This means that a candidate key is a set of one or more attributes (columns) that uniquely identifies a tuple (row) in a table, and it has the least number of attributes needed to do so. In a nutshell, the candidate key is the minimal set of super keys.
On the other hand, a super key is any set of one or more attributes that can be used to uniquely identify a tuple in a table, regardless of whether it is minimal or not. It means that a super key can have more attributes than a candidate key.
For example, consider a table that has three columns: "employee_id", "name", and "email". The "employee_id" column can be used as a candidate key because it uniquely identifies each row in the table. However, the set {"employee_id", "email"} is also a super key because it also uniquely identifies each row in the table.
In summary, a super key is any set of columns that can uniquely identify a row in a table, while a candidate key is a super key with the minimum number of attributes that can be used to uniquely identify a row in a table.
| Super Key | Candidate Key |
|---|---|
| Super set of all such attributes that can uniquely identify the table, it is not compulsory that all super keys are candidate keys | All the candidate keys are super keys or part of the Super key |
| It can be NULL, which means its values can be NULL | It can never be NULL, which means its values cannot be a NULL |
| super set of candidate key | Subset of super key |
How to Find a Candidate Key in DBMS?
There are several methods to find a candidate key in DBMS. Some of them are listed below.
-
Method 1 to find candidate key in DBMS:
In this method, we look for a column or a set of columns that can be used to uniquely identify a tuple in a table. This can be done by checking the values of each column and determining if they are unique across the entire table. -
Method 2 to find candidate key in DBMS:
Another method is to use the functional dependencies (FDs) among the attributes of a table. A functional dependency is a relationship between two attributes where one attribute is functionally dependent on the other. For example, if a table has columns "student_ID" and "student_name", there is a functional dependency between "student_ID" and "student_name", as the name is determined by the ID. Using functional dependencies, we can find the candidate key by identifying the set of attributes that determine all other attributes in the table. -
Method 3 to find candidate key in DBMS:
This method uses the normal forms (1NF, 2N, 3NF, etc.) to identify the candidate key. Normal forms are a set of guidelines for designing a relational database and can be used to identify the candidate key. For example, a table that is in 2nd normal form must have a primary key, which is a candidate key. -
Method 4 to find candidate key in DBMS:
Lastly, some DBMS software, like MySQL, have inbuilt functions like "SHOW KEYS" that can be used to list all indexes, including primary keys and candidate keys, on a specific table.Once a candidate key is found, it can be used as a primary key to uniquely identify a tuple in a table and to reference the tuple in other tables through foreign keys.
Conclusion
In conclusion, a candidate key is a set of one or more columns in a table that can be used to uniquely identify a tuple in a table. It is a minimal set of columns that, together, can be used to differentiate one tuple from another. A primary key is a candidate key that is chosen to be the main way to identify a tuple in a table. It is a special candidate key that is used to uniquely identify each row in a table, and it cannot contain null values. The primary key is used to reference the tuple in other tables through a foreign key, which is an attribute in another table that references the primary key in the table.
Frequently Asked Questions(FAQs)
Here are some frequently asked questions on candidate key in DBMS.
Ques 1. How are candidate keys different from primary keys in DBMS?
Ans. A candidate key is a set of attributes that can uniquely identify each record in a table, while a primary key is the selected candidate key used to uniquely identify each record in that table.
Ques 2. Can a table have more than one candidate key?
Ans. Yes, a table can have multiple candidate keys.
Ques 3. Can a candidate key contain null values?
Ans. No, a candidate key cannot contain null values as it must be able to uniquely identify each record in the table.
Ques 4. Can a foreign key be a candidate key?
Ans. Yes, a foreign key can be a candidate key if it is a set of attributes that can uniquely identify each record in a table.
Ques 5. How can you identify candidate keys in a table?
Ans. Candidate keys in DBMS can be identified by analyzing the functional dependencies between the attributes in a table and selecting the minimal set of attributes that can uniquely identify each.