What are keys? Discuss its types.
Subject
Database Management System
Standard
Computer Science Engineering
Views
584
Asked By
Deepak
Answer / Solution
In a database table, a key is a column or combination of columns that uniquely identify each row in the table.
There are several types of keys that can be used in database tables, including:
- Primary Key: A primary key is a column or combination of columns that uniquely identifies each row in a table. It is typically used as the main way to identify and relate records to each other.
- Foreign Key: A foreign key is a column or combination of columns that references the primary key of another table. It is used to establish relationships between tables.
- Candidate Key: A candidate key is a column or combination of columns that could potentially serve as the primary key of a table, but is not currently being used as such.
- Unique Key: A unique key is a column or combination of columns that uniquely identifies each row in a table, similar to a primary key. However, unlike a primary key, a table can have multiple unique keys.
- Composite Key: A composite key is a combination of two or more columns that uniquely identifies each row in a table.
Keys are important in a database because they help to ensure data integrity and consistency, and they facilitate efficient querying and manipulation of data.
Answer / Solution
Primary key
A primary key is a column or a set of columns that uniquely identifies each row in a table. It cannot have null values, and each value must be unique. A table can have only one primary key.
- It is the first key used to identify one and only one instance of an entity uniquely. An entity can contain multiple keys, as we saw in the PERSON table. The key which is most suitable from those lists becomes a primary key.
- In the EMPLOYEE table, ID can be the primary key since it is unique for each employee. In the EMPLOYEE table, we can even select License_Number and Passport_Number as primary keys since they are also unique.
- For each entity, the primary key selection is based on requirements and developers.
Unique key:
A unique key is a column or a set of columns that must have unique values, but unlike a primary key, it can have null values.
Candidate key
A candidate key is a column or a set of columns that could potentially be used as a primary key. It must also be unique and cannot have null values.
- A candidate key is an attribute or set of attributes that can uniquely identify a tuple.
- Except for the primary key, the remaining attributes are considered a candidate key. The candidate keys are as strong as the primary key.
For example: In the EMPLOYEE table, id is best suited for the primary key. The rest of the attributes, like SSN, Passport_Number, License_Number, etc., are considered a candidate key.
Super Key
Super key is an attribute set that can uniquely identify a tuple. A super key is a superset of a candidate key.
For example: In the above EMPLOYEE table, for(EMPLOEE_ID, EMPLOYEE_NAME), the name of two employees can be the same, but their EMPLYEE_ID can't be the same. Hence, this combination can also be a key.
The super key would be EMPLOYEE-ID (EMPLOYEE_ID, EMPLOYEE-NAME), etc.
Foreign key
A foreign key is a column or a set of columns in one table that refers to the primary key of another table. It is used to establish a relationship between two tables.
- Foreign keys are the column of the table used to point to the primary key of another table.
- Every employee works in a specific department in a company, and employee and department are two different entities. So we can't store the department's information in the employee table. That's why we link these two tables through the primary key of one table.
- We add the primary key of the DEPARTMENT table, Department_Id, as a new attribute in the EMPLOYEE table.
- In the EMPLOYEE table, Department_Id is the foreign key, and both the tables are related.
Alternate key
There may be one or more attributes or a combination of attributes that uniquely identify each tuple in a relation. These attributes or combinations of the attributes are called the candidate keys. One key is chosen as the primary key from these candidate keys, and the remaining candidate key, if it exists, is termed the alternate key.
In other words, the total number of the alternate keys is the total number of candidate keys minus the primary key. The alternate key may or may not exist. If there is only one candidate key in a relation, it does not have an alternate key.
For example, employee relation has two attributes, Employee_Id and PAN_No, that act as candidate keys. In this relation, Employee_Id is chosen as the primary key, so the other candidate key, PAN_No, acts as the Alternate key.
Composite key
Whenever a primary key consists of more than one attribute, it is known as a composite key. This key is also known as Concatenated Key. A composite key is a key that consists of two or more columns in a table that together uniquely identify each row.
For example, in employee relations, we assume that an employee may be assigned multiple roles, and an employee may work on multiple projects simultaneously. So the primary key will be composed of all three attributes, namely Emp_ID, Emp_role, and Proj_ID in combination. So these attributes act as a composite key since the primary key comprises more than one attribute.
Artificial key
The key created using arbitrarily assigned data are known as artificial keys. These keys are created when a primary key is large and complex and has no relationship with many other relations. The data values of the artificial keys are usually numbered in a serial order.
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