What is a JOIN in databases, and what types are there?

JOIN is an operation in relational databases that allows combining rows from two or more tables based on a related column.

Main types of JOIN:

• INNER JOIN — returns only the records with matches in both tables.
• LEFT JOIN (LEFT OUTER JOIN) — returns all records from the left table and matching records from the right; if there are no matches, NULLs will appear on the right.
• RIGHT JOIN (RIGHT OUTER JOIN) — the opposite, returns all from the right table and matching from the left.
• FULL JOIN (FULL OUTER JOIN) — returns all records from both tables, filling NULLs where there are no matches.

Example in SQL:

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

This query will return the names of users and their orders, only for users who have orders.