What Are Relational Databases?
A relational database is a type of database that stores and organizes data in a structured way. It uses a structure that allows data to be identified and accessed in relation to other data in the database. Data in a relational database is stored in various data tables, each of which has a unique key identifying every row.
Relational databases are made up of a set of tables with data that fits into a predefined category. Each table has at least one data category in a column, and each row contains a certain data instance for the categories defined in the columns.
For example, consider an 'Employees' table:
| EmployeeId | FirstName | LastName | |
|---|---|---|---|
| 1 | John | Doe | john.doe@example.com |
| 2 | Jane | Doe | jane.doe@example.com |
| 3 | Bob | Smith | bob.smith@example.com |
In this table, 'EmployeeId', 'FirstName', 'LastName' and 'Email' are categories, and each row represents a specific employee.
Relationships
The term "relational database" comes from the concept of a relation—a set of tuples that the database organizes into rows and columns. Each row in a table represents a relationship among a set of values.
Relational databases use keys to create links between tables. A primary key is a unique identifier for a row of
data. A foreign key is a column or combination of columns used to establish and enforce a link between the data in two
tables.
Consider an additional 'Orders' table:
| OrderId | EmployeeId | Product |
|---|---|---|
| 1 | 3 | Apples |
| 2 | 1 | Bananas |
| 3 | 2 | Cherries |
In the 'Orders' table, 'EmployeeId' serves as the foreign key creating a relationship between 'Orders' and 'Employees'. This allows queries that involve data in both tables, like "Find all orders placed by John Doe".
SELECT Orders.OrderId, Orders.Product, Employees.FirstName, Employees.LastName
FROM Orders
INNER JOIN Employees ON Orders.EmployeeId = Employees.EmployeeId;
The above SQL code is an example of how to retrieve data from a relational database using a JOIN clause to combine
rows from two or more tables.
Overall, relational databases provide a powerful mechanism for defining relationships within data and enabling efficient data retrieval.