Description: A subquery is a nested query within another SQL query. It is used to perform complex operations on databases, allowing one query to depend on the results of another. Subqueries can appear in various parts of an SQL query, such as in the SELECT, WHERE, or FROM clauses, and are fundamental for data manipulation and retrieval in database management systems. Their use simplifies complex queries, improves code readability, and optimizes performance by reducing the need for multiple independent queries. Subqueries can return a single value, a set of values, or even a set of records, making them versatile for different query scenarios. Additionally, they are compatible with various relational database management systems, where they can be integrated into SQL queries to facilitate database interaction.
Uses: Subqueries are primarily used to filter results, calculate aggregates, or perform complex joins between tables. They are especially useful in situations where a query needs to be based on the result of another, such as retrieving all employees whose salary is above the average of their department. They are also employed in query optimization, allowing developers to structure their queries more efficiently and readably.
Examples: An example of a subquery is: ‘SELECT name FROM employees WHERE salary > (SELECT AVG(salary) FROM employees WHERE department_id = 1);’, which returns the names of employees whose salary is greater than the average of their department. Another case is the use of subqueries in the FROM clause, such as in ‘SELECT * FROM (SELECT name, salary FROM employees) AS subquery WHERE salary > 50000;’, which allows working with an intermediate result set.
