A key in a DBMS (Database Management System) is a field or set of fields that uniquely identifies a record (row) in a table. Keys are used to enforce data integrity, ensure data consistency, and provide a way to retrieve data quickly and efficiently. In this tutorial, we will discuss the different types of keys in a DBMS, their properties, and how to define them.
- Primary Key A primary key is a field or a set of fields that uniquely identify each record in a table. Primary keys can consist of one or more columns and are defined when a table is created. A primary key cannot be null and must be unique for each record in the table. Primary keys are used to enforce data integrity and to provide a way to uniquely identify a record.
Let’s consider a table named “Students” with the following columns: StudentID (primary key), FirstName, LastName, and Age. The StudentID column is unique and not null, and it uniquely identifies each record in the table.
2. Foreign Key A foreign key is a field or set of fields in one table that refers to the primary key in another table. Foreign keys are used to create relationships between tables and to ensure data consistency. When a foreign key is defined in a table, it must reference a valid primary key in another table. Foreign keys can also be used to enforce referential integrity, which ensures that data in related tables is consistent.
Let’s consider another table named “Courses” with the following columns: CourseID (primary key), CourseName, and InstructorID (foreign key). The InstructorID column in the Courses table references the primary key (InstructorID) of the Instructors table, creating a relationship between the two tables.
3. Candidate Key A candidate key is a field or a set of fields that could be used as a primary key. Candidate keys are unique and not null, but they are not necessarily chosen as the primary key. A table can have multiple candidate keys, but only one primary key.
Let’s say we have a table named “Employees” with the following columns: EmployeeID (primary key), Email, and Phone. Both the Email and Phone columns are unique and not null, so they could be used as candidate keys.
Super key is an attribute set that can uniquely identify a tuple. A super key is a superset of a candidate key.
n 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.
4. Alternate Key An alternate key is a field or a set of fields that is unique and not null, but it is not a candidate key or a primary key. Alternate keys are sometimes used for indexing and searching data.
Continuing with the “Employees” table example, suppose we add a new column called “BadgeNumber” which is also unique and not null. This column could be used as an alternate key for indexing and searching the data.
5. Composite Key A composite key is a primary key that consists of two or more fields. A composite key is used when a single field cannot uniquely identify a record, but the combination of multiple fields can. For example, a composite key may consist of a customer ID and a date, which together uniquely identify a customer order.
Let’s consider a table named “Orders” with the following columns: OrderID (primary key), CustomerID, and OrderDate. The combination of CustomerID and OrderDate uniquely identifies each record in the table, making it a composite key.
6. Super Key A super key is a combination of fields that uniquely identify a record, but it may include additional fields that are not necessary for unique identification. Super keys are used to identify a set of records that satisfy a specific condition, but they are not necessarily unique.
Let’s consider a table named “Inventory” with the following columns: ProductID, ProductName, Quantity, and Price. The combination of ProductID, ProductName, and Quantity uniquely identifies each record in the table, but the Price column is not necessary for unique identification. Therefore, the combination of all four columns is a super key.
In conclusion, keys are an essential part of a DBMS, and their proper use ensures data integrity, consistency, and retrieval efficiency. When designing a database, it is important to choose the appropriate type of key for each table and to define them correctly to ensure optimal performance and data quality.