Welcome to subqueries! These are queries inside other queries - like nesting one SELECT statement inside another. Think of them as asking SQL to "first find this, then use that result to find something else."
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When might you need to find data based on results from another query?
A subquery always goes inside parentheses and typically appears in the WHERE clause. The inner query runs first, returns results, then the outer query uses those results.
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Which part runs first—inner SELECT or outer SELECT?
Let's break down that example. The subquery (SELECT customer_id FROM orders)
finds all customer IDs who have placed orders. Then the main query finds names for those customers.
This gives you customers who have actually ordered something.
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Why might finding "customers with orders" be useful for a business?
Here's another practical example. Find products that cost more than the average price:
The subquery calculates the average first, then the main query finds products above that average.
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What does the subquery (SELECT AVG(price) FROM products)
return?
Subqueries can also appear in FROM clauses, creating temporary result sets. This treats the subquery results like a table:
