Joins combine rows from two or more tables based on related columns. Here are the main types.

INNER JOIN

Returns only matching rows from both tables:

SELECT employees.name, departments.department_name
FROM employees
INNER JOIN departments ON employees.department_id = departments.id;

Only employees with a matching department will appear in results.

LEFT JOIN (LEFT OUTER JOIN)

Returns all rows from the left table, with matching rows from the right:

SELECT employees.name, departments.department_name
FROM employees
LEFT JOIN departments ON employees.department_id = departments.id;

All employees appear, even those without a department (NULL for department_name).

RIGHT JOIN (RIGHT OUTER JOIN)

Returns all rows from the right table, with matching rows from the left:

SELECT employees.name, departments.department_name
FROM employees
RIGHT JOIN departments ON employees.department_id = departments.id;

All departments appear, even those without employees.

FULL JOIN (FULL OUTER JOIN)

Returns all rows when there’s a match in either table:

SELECT employees.name, departments.department_name
FROM employees
FULL JOIN departments ON employees.department_id = departments.id;

All employees and all departments appear, with NULLs where no match exists.

Multiple Joins

Join more than two tables:

SELECT 
    employees.name,
    departments.department_name,
    locations.city
FROM employees
INNER JOIN departments ON employees.department_id = departments.id
INNER JOIN locations ON departments.location_id = locations.id;

Using Table Aliases

Make queries more readable:

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

Self Join

Join a table to itself:

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

Summary

Join Type	Returns
INNER	Only matching rows
LEFT	All left + matching right
RIGHT	All right + matching left
FULL	All rows from both tables

Choose the join type based on whether you need all rows from one or both tables.

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