What is a JOIN? What types of JOINs exist and what are their differences?

JOIN is an operation in SQL that allows you to combine rows from two or more tables based on a related condition.

Main types of JOIN:

• INNER JOIN — returns only those records that have 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 are shown 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 in NULLs where there are no matches.
• CROSS JOIN — Cartesian product, returns all possible combinations of rows from both tables.

Differences in which rows are included in the result when there are no matches:

• INNER JOIN excludes them.
• LEFT/RIGHT JOIN includes all from one table and matching from the other.
• FULL JOIN includes all from both.

Example of INNER JOIN:

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

This query will return users who have orders.