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Part 2: Relational Databases (RDBMS / PostgreSQL / MySQL) (6 - 25)

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Databases & System Design•Part 2: Relational Databases (RDBMS / PostgreSQL / MySQL) (6 - 25)

SQL Joins comparison:

In Simple Words:

  • INNER JOIN: Returns rows only when keys match in both tables.
  • LEFT JOIN: Returns all left rows plus matched right rows (NULL if no match).
  • CROSS JOIN: Cartesian product ($M \times N$ rows; dangerous on large tables).
sql
-- LEFT JOIN: Find all users, including those who have never placed an order:
SELECT u.id, u.name, o.id AS order_id
FROM users u
LEFT JOIN orders o ON u.id = o.user_id
WHERE o.id IS NULL; -- Users with zero orders
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Join Execution Algorithms (Nested Loop, Hash Join, Merge Join):