What is JOIN in a database and how does LEFT JOIN work?

JOIN in a database is an operation that allows you to combine rows from two or more tables based on a related condition (usually by key). This enables you to retrieve combined data from multiple tables in a single query.

LEFT JOIN is a type of join that returns all rows from the left table and the matching rows from the right table. If there are no matches in the right table, the result set will contain NULL values for the right table.

Example:

SELECT users.id, users.name, orders.id AS order_id
FROM users
LEFT JOIN orders ON users.id = orders.user_id;

In this example, all users will be returned, even if they have no orders. For users without orders, the order_id field will be NULL.