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What is an INNER JOIN, and how does it combine data from two tables? How do you find rows that exist in one table but have no matching row in another table using a join? What is a FULL OUTER JOIN, and when would you use one instead of a LEFT or RIGHT JOIN? What common mistakes do developers make when writing multi table joins that can lead to unexpectedly large or incorrect result sets? How does a CROSS JOIN work, and what are some legitimate use cases for it despite the risk of accidentally creating a huge result set? How does SQL Server's query optimizer decide which physical join algorithm, such as nested loops, merge join, or hash join, to actually use for a given query?
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What is the difference between a LEFT JOIN and a RIGHT JOIN?

Beginner

Answer

A LEFT JOIN returns all rows from the left table along with any matching rows from the right table, filling in NULL for columns from the right table when no match exists, while a RIGHT JOIN does the same thing but reversed, keeping all rows from the right table instead.
SELECT c.Name, o.OrderId
FROM Customers c
LEFT JOIN Orders o ON c.CustomerId = o.CustomerId;
Real-world example A customer report uses a LEFT JOIN to show every single customer, including those who have never placed an order, displaying a NULL order id for customers who have not made a purchase yet.

Common follow-ups: Why do most developers prefer to always use LEFT JOIN instead of RIGHT JOIN for consistency?;How do you find customers who have never placed an order using this kind of join?

Aggregate Functions & GROUP BY;Subqueries
All 7 Joins questions