SQL LEFT JOIN

SQL LEFT JOIN returns all records from the left table (table1) and the matched records from the right table (table2). If no match is found in the right table, NULL values are returned for the corresponding columns.

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1. Overview

SQL LEFT JOIN is used to retrieve data from two tables based on a related column, with all the rows from the left table and matching rows (if any) from the right table. It ensures that all records from the left table are included in the result set, even if there is no matching record in the right table.

Example:

// Example of using LEFT JOIN to retrieve data from two tables
SELECT customers.customer_name, orders.order_date
FROM customers
LEFT JOIN orders ON customers.customer_id = orders.customer_id;

This example retrieves the customer names along with their order dates, ensuring that all customers from the customers table are included in the result set, regardless of whether they have placed any orders.

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2. Syntax

The syntax for SQL LEFT JOIN is as follows:

SELECT columns
FROM table1
LEFT JOIN table2 ON table1.column = table2.column;

Where table1 is the left table, table2 is the right table, and column is the related column that exists in both tables.

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3. Usage

SQL LEFT JOIN is commonly used to retrieve data from a primary table (such as customers or employees) along with related data from another table (such as orders or transactions). It ensures that all records from the primary table are included in the result set, regardless of whether there is matching data in the related table.

Example:

// Example of using LEFT JOIN to retrieve all customers and their orders
SELECT customers.customer_name, orders.order_date
FROM customers
LEFT JOIN orders ON customers.customer_id = orders.customer_id;

This example retrieves all customer names along with their order dates, even if some customers have not placed any orders.

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4. Conclusion

SQL LEFT JOIN is a valuable tool for retrieving data from multiple tables while ensuring that all records from the primary table are included in the result set. It enables comprehensive data analysis and reporting by combining related data from different tables in a database.