A query in a relational database is a request for data or information that retrieves, modifies, or interacts with the data stored in the database. Typically, queries are written in Structured Query Language (SQL), which is a standardized language specifically designed for managing and manipulating relational databases. A query can fetch specific records, update existing records, insert new data, or delete records, depending on what the user needs. For example, a simple SQL query to retrieve the names of all employees from an "employees" table might look like this: SELECT name FROM employees;.
Queries can be straightforward, like retrieving all records from a table, or more complex, involving multiple tables through joins, filters, and sorting. For instance, if a developer wants to retrieve the names and salaries of employees in a specific department, they might use a more intricate query combining the "employees" and "departments" tables, such as:
SELECT e.name, e.salary
FROM employees e
JOIN departments d ON e.department_id = d.id
WHERE d.name = 'Sales';
In this example, the query not only retrieves data but also filters based on a specific condition.
Moreover, queries also include functionality for data manipulation and management. Developers can use SQL commands like INSERT, UPDATE, and DELETE to add new records, change existing data, or remove data altogether. For example, to add a new employee, a developer would execute an INSERT statement like:
INSERT INTO employees (name, salary, department_id)
VALUES ('John Doe', 60000, 1);
Thus, queries are fundamental to interacting with the data in relational databases, allowing developers to efficiently perform a variety of operations tailored to their application's needs.