Detailed explanation of nested query instances in MySQL database

Nested queries are a way to include another query in one query. They are mainly used to retrieve data that meets complex conditions, associate multiple tables, and calculate summary values ​​or statistical information. Examples include finding employees above average wages, finding orders for a specific category, and calculating the total order volume for each product. When writing nested queries, you need to follow: write subqueries, write their results to outer queries (using alias or AS clause references), and optimize query performance (using indexes).

Detailed explanation of nested query instances in MySQL database

What is nested query?

A nested query is to include another query in one query. The outer query uses the results of the inner query as part of its conditions.

Scenarios using nested queries

Nested queries are mainly used in the following scenarios:

• Retrieve data that meets complex conditions
• Related data from multiple tables
• Calculate summary values ​​or statistics

Nested query instances

Example 1: Find Employee Information

 <code class="sql">SELECT * FROM employees WHERE salary > ( SELECT AVG(salary) FROM employees );</code>

This query looks for all employees who have a salary above the average salary.

Example 2: Find order information

 <code class="sql">SELECT * FROM orders WHERE product_id IN ( SELECT product_id FROM products WHERE category = 'Electronics' );</code>

This query finds orders related to electronic products.

Example 3: Calculate summary value

 <code class="sql">SELECT product_id, SUM(quantity) AS total_quantity FROM order_details GROUP BY product_id;</code>

This query calculates the total order quantity for each product.

How to write nested queries

When writing nested queries, follow these steps:

1. Write an inner layer query, that is, a subquery.
2. Write the results of the inner query as part of the condition and write it to the outer query.
3. Use the alias of the subquery or the AS clause to reference the results of the subquery.

Things to note

• Nested queries can degrade performance, especially when inner queries contain large amounts of data.
• Carefully consider the logic of nested queries to ensure that the expected results are returned.
• Use indexes to optimize performance of nested queries.

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