SQL Subqueries

SQL Subqueries are queries nested within another query, allowing you to use the result of one query as a condition or value in another query.

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1. Overview

SQL Subqueries are queries that are embedded within the WHERE clause, FROM clause, HAVING clause, or SELECT statement of another SQL query. They can be used to filter results, perform calculations, or retrieve specific data based on conditions.

Example:

// Example of using a subquery to filter results
SELECT customer_name
FROM customers
WHERE customer_id IN (SELECT customer_id FROM orders WHERE order_date = '2024-04-10');

This example retrieves the names of customers who placed orders on a specific date by using a subquery to filter the customer IDs.

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2. Syntax

The syntax for SQL Subqueries varies depending on where they are used:

• Subquery in WHERE clause: SELECT column(s) FROM table WHERE column IN (SELECT column FROM table);
• Subquery in FROM clause: SELECT column(s) FROM (SELECT column FROM table) AS subquery_table;
• Subquery in HAVING clause: SELECT column(s) FROM table GROUP BY column HAVING aggregate_function(column) IN (SELECT column FROM table);
• Subquery in SELECT statement: SELECT column, (SELECT column FROM table) AS subquery_column FROM table;

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3. Usage

SQL Subqueries are commonly used to:

• Filter results based on conditions that cannot be expressed using simple WHERE clauses.
• Perform calculations or aggregate functions on subsets of data.
• Retrieve values from related tables or tables with complex structures.

Example:

// Example of using a subquery to calculate the average order amount
SELECT customer_id, (SELECT AVG(order_amount) FROM orders WHERE orders.customer_id = customers.customer_id) AS avg_order_amount
FROM customers;

This example calculates the average order amount for each customer using a subquery within the SELECT statement.

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4. Conclusion

SQL Subqueries are a powerful feature that allows you to perform complex operations and retrieve specific data within SQL queries. By nesting queries within each other, you can manipulate and analyze data in various ways to meet your requirements.