Joins and relationships

6 lessons in the SQL course.

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• Primary KeysGive every row a stable, unique identity with a primary key.
• Foreign KeysConnect tables and protect references with foreign key constraints.
• INNER JOINCombine rows from related tables when both sides match.
• LEFT JOINKeep every left-side row even when no related row exists.
• COUNT, SUM, and AVGSummarize sets of rows with common aggregate functions.
• Grouping ResultsProduce one aggregate result for each category with GROUP BY.