Explain the differences between INNER JOIN and LEFT JOIN operations in SQL and their application in queries.

INNER JOIN returns only those rows that have matches in both tables based on the join condition. If there are no matches, the row will not be included in the result.

LEFT JOIN (or LEFT OUTER JOIN) returns all rows from the left table and the matching rows from the right table. If there are no matches, the columns from the right table will contain NULL.

Example:

SELECT users.id, orders.id
FROM users
INNER JOIN orders ON users.id = orders.user_id;

Returns only users with orders.

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

Returns all users, including those without orders (in this case, orders.id will be NULL).

Usage:

• INNER JOIN — when only related data is needed.
• LEFT JOIN — when you want to keep all data from the left table and add related data if it exists.