In short
SQL's mechanism for combining rows from multiple tables based on a related column — essential for querying data that's spread across your normalized schema.
How to learn this
- 1Practice INNER JOIN, LEFT JOIN and their different results on the same tables.
- 2Write a query joining 3+ tables to answer a real question, e.g. 'orders with customer and product names'.
- 3Learn when a JOIN is more efficient than multiple separate queries.
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