Subqueries
Subqueries are SQL queries nested within another SQL query, allowing for complex data retrieval by using the result of an inner query as input for an outer query. They are commonly used in SELECT, INSERT, UPDATE, and DELETE statements to filter, compare, or aggregate data based on dynamic conditions derived from the database itself. This enables more sophisticated and efficient data manipulation without requiring multiple separate queries.
Developers should learn subqueries when working with relational databases to handle scenarios like filtering results based on aggregated values (e.g., finding employees with salaries above the department average), performing existence checks (e.g., using EXISTS to verify related records), or simplifying complex joins. They are essential for writing concise, readable SQL code in applications involving data analysis, reporting, or transactional systems where dynamic data dependencies are common.