In this section 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
A candidate key is a portion of the super key that is able to separately identify the other table properties. There may be more than one potential key for a table. The candidate key assists in identifying a table’s prime and non-prime attributes and protects the data’s integrity by avoiding data duplication. As a result, the candidate key is a crucial idea when constructing a database schema.
Define Key in DBMS
In a database management system (DBMS), a key is a specific value or set of values within a table that is used to uniquely identify each record or row within that table. There are several types of keys used in DBMS, including primary keys, foreign keys, and candidate keys. 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 ?
A candidate key 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 can have multiple candidate keys, but at least one is required. A candidate key that is chosen to uniquely identify the tuples in a relation is called the primary key.
Define Candidate Key in DBMS:
A candidate key in a database management system (DBMS) is a set of one or more attributes that uniquely identifies a tuple (i.e., a row) in a relation (i.e., table). In other words, it is a column or set of columns that can be used to identify each row in a table. A relation can have multiple candidate keys, and one of them is chosen as the primary key, which is used to enforce the integrity constraints of the relation and to create relationships with other relations. 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:
In this example, we have a student details table that consists 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 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 include:
- Uniqueness: Each tuple in a relation can be uniquely identified by its candidate key, which 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: The use of candidate keys enforces the rule of "entity integrity," which states that each tuple in a relation must have a unique identifier. This helps to ensure that the data in the relation is accurate and consistent.
- 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?
In database management systems, a super key is a set of one or more attributes (columns) that uniquely identify a tuple (row) in 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.
Difference between Super Key and Candidate 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 simple the minimal set of super key is called candidate key
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.
Candidate Key vs Super 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 Candidate Key in DBMS?
There are several methods to find a candidate key in a database management system (DBMS):
-
The first method is to look for 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.
-
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.
-
Another method is to use 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.
-
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.
Summary
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. For example, a table of student information might have a candidate key of Roll_no, as each student has a unique ID.
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.
In summary, a candidate in a DBMS refers to a tuple that satisfies a specific condition in a query, and a candidate key is a minimal set of columns that can be used to uniquely identify a tuple in a table. A primary key is a candidate key that is chosen to be the main way to identify a tuple in a table, and it is used to reference the tuple in other tables through a foreign key.