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What is the difference between a LEFT JOIN and a RIGHT JOIN? 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 an INNER JOIN, and how does it combine data from two tables?

Beginner

Answer

An INNER JOIN combines rows from two tables based on a matching condition, returning only the rows where a match is found in both tables, meaning any row without a corresponding match in the other table is simply left out of the results entirely.
SELECT o.OrderId, c.Name
FROM Orders o
INNER JOIN Customers c ON o.CustomerId = c.CustomerId;
Real-world example An order report combines order details with customer names using an INNER JOIN, showing only orders that are linked to an actual existing customer record, and excluding any orphaned data if it somehow existed.

Common follow-ups: What happens to rows that do not have a match when using an INNER JOIN?;Can you join more than two tables together in a single query?

Constraints (Primary Key Foreign Key Check & Unique);Data Types & Schema Design
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