JOINs (INNER, LEFT, RIGHT)

JOINs combine rows from two tables using a related column. INNER JOIN returns only rows that match in both tables. LEFT JOIN returns every row from the left table, with NULL where there's no match. RIGHT JOIN is the same idea but keeps every row from the right table.

-- Users who have posts
SELECT users.name, posts.title
FROM users
INNER JOIN posts ON posts.user_id = users.id;

-- All users, even ones without posts (title will be NULL)
SELECT users.name, posts.title
FROM users
LEFT JOIN posts ON posts.user_id = users.id;

-- Short aliases
SELECT u.name, COUNT(p.id) AS post_count
FROM users u
LEFT JOIN posts p ON p.user_id = u.id
GROUP BY u.name;

JOINs (INNER, LEFT, RIGHT)

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