In SQL, a nested query, also known as a subquery, is a query inside another query. A nested query is typically used to retrieve more specific data based on the results of the outer query. In other words, a nested query is a query that is nested inside another query.

The basic syntax for a nested query is as follows:

SELECT [column1], [column2], ...
FROM [table1]
WHERE [column] [operator] (SELECT [column] FROM [table] WHERE [condition]);

In this syntax, the subquery is enclosed in parentheses and is used in place of the condition that would normally be used in the WHERE clause.

Let’s take a look at some examples to better understand how nested queries work.

Example 1:

Suppose we have a database with two tables, “customers” and “orders”. The “customers” table has columns “customer_id”, “customer_name”, and “customer_email”, while the “orders” table has columns “order_id”, “customer_id”, and “order_date”.

Suppose we want to retrieve the names of all customers who have placed an order in the past month. We can accomplish this using a nested query:

SELECT customer_name
FROM customers
WHERE customer_id IN (SELECT customer_id
                      FROM orders
                      WHERE order_date >= DATEADD(month, -1, GETDATE()))

In this query, the nested query retrieves all the customer IDs of customers who have placed an order in the past month. The outer query then retrieves the customer names of those customers whose IDs were retrieved by the nested query.

Example 2:

Suppose we have a database with two tables, “employees” and “departments”. The “employees” table has columns “employee_id”, “employee_name”, and “department_id”, while the “departments” table has columns “department_id” and “department_name”.

Suppose we want to retrieve a list of all departments and the names of their highest-paid employees. We can accomplish this using a nested query:

SELECT d.department_name, e.employee_name
FROM employees e
INNER JOIN departments d ON e.department_id = d.department_id
WHERE e.employee_id IN (SELECT TOP 1 employee_id
                        FROM employees
                        WHERE department_id = d.department_id
                        ORDER BY salary DESC)

In this query, the nested query retrieves the ID of the highest-paid employee in each department. The outer query then retrieves the department name and the name of the employee with the ID retrieved by the nested query.

Nested queries can be used in many different ways in SQL, including in SELECT, WHERE, and HAVING clauses. They are a powerful tool that can be used to retrieve specific data that may be difficult to retrieve with a single query. However, they can also be computationally expensive and can slow down the performance of the database if used excessively. It is important to use nested queries judiciously and to optimize their performance whenever possible.

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