SQL JOINs Explained: INNER, LEFT, FULL OUTER, SELF, and CROSS JOIN

Normalized data often stores related facts in separate tables. Employees can live in one table, departments in another, and orders in another. JOINs let you bring those facts together when you query.

SQL for Files lets you practice JOINs in the browser with sample tables before trying the same patterns on your own local files.

INNER JOIN returns rows where the join condition matches on both sides. If an employee has no matching department, that employee is excluded.

SELECT e.name AS employee_name, d.name AS department_name
FROM employees AS e
INNER JOIN departments AS d
  ON e.department_id = d.id;

LEFT JOIN keeps every row from the left table and uses NULL when the right side has no match. FULL OUTER JOIN keeps unmatched rows from both sides.

SELECT d.name AS department_name, e.name AS employee_name
FROM departments AS d
LEFT JOIN employees AS e
  ON d.id = e.department_id;

A SELF JOIN connects rows in a table to other rows in the same table, such as employees to managers. A CROSS JOIN creates every possible combination between two tables, such as departments and meeting days.

SELECT e.name AS employee_name, m.name AS manager_name
FROM employees AS e
LEFT JOIN employees AS m
  ON e.manager_id = m.id;

1. Start with why JOINs exist and how keys connect tables.
2. Use INNER JOIN for matching rows and LEFT JOIN for preserving one side.
3. Find missing matches with LEFT JOIN and IS NULL.
4. Finish with FULL OUTER, SELF, and CROSS JOIN patterns.