sql subquery

.sql subquery

SQL subqueries are queries nesting inside another query. They are defined as queries that are included in a main query, the parent query. Subqueries return a single value, a single row or multiple rows to the parent query. Subqueries are written within parentheses and nested inside the parentheses required for the main query.

Types of Subqueries

There are mainly two types of subqueries: single-row subquery and multi-row subquery. Single-row subqueries return a single row, while multi-row subqueries return multiple rows. A single-row subquery can only be used with a comparison operator that expects a single value, such as =, , >=, or <=. A multi-row subquery can be used with comparison operators that expect multiple values, such as IN, ANY, or ALL.

Uses of Subqueries

Subqueries can be used in many ways. They can be used in the WHERE clause to filter records based on data from another table. They can be used in the SELECT statement to return a calculated value based on data from another table. They can be used in the FROM clause to create a derived table that is used in the main query. They can also be used in the HAVING clause to filter records based on aggregate functions.

Examples of Single-row Subquery

An example of a single-row subquery is as follows: SELECT * FROM employees WHERE salary = (SELECT MAX(salary) FROM employees). This query selects all the information from the employees table where the salary is equal to the maximum salary of all the employees. The subquery returns a single value, the maximum salary from the employees table, which is used to filter the records in the main query.

Examples of Multi-row Subquery

An example of a multi-row subquery is as follows: SELECT department_id, department_name FROM departments WHERE department_id IN (SELECT department_id FROM employees GROUP BY department_id HAVING COUNT(*) > 5). This query selects the department ID and department name from the departments table where the department ID is in the result of a subquery that selects the department ID from the employees table and groups the results by department ID. The subquery returns all the department IDs where there are more than five employees, which is used to filter the records in the main query.