Keys, normalization & good design
A primary key identifies a row; a foreign key links to another table. Normalization reduces duplicated facts. Keep customer details in customers and reference customer_id from orders. Check performance and real requirements before redesigning.
हिन्दी में समझें
प्राथमिक कुंजी पंक्ति पहचानती है; विदेशी कुंजी दूसरी टेबल जोड़ती है। नॉर्मलाइज़ेशन दोहराई जानकारी घटाता है। ग्राहक जानकारी customers और संदर्भ orders में रखें।
अभ्यास: हर ग्राहक id, name और orders की संख्या total दिखाएँ, शून्य order वाले भी; id क्रम।
Hinglish mein samjhein
Primary key row identify karti hai; foreign key doosri table link karti hai. Normalization repeated facts kam karta hai. Customer details customers mein, customer_id orders mein rakho.
Practice: Har customer id,name aur order count total dikhao, zero-order bhi; customer id sorted.
A small SQL example
SELECT o.id,c.name,c.city FROM orders o JOIN customers c ON c.id=o.customer_id ORDER BY o.id;Examples use the fictional shop database bundled with the SQL playground: customers, products, orders, order_items and employees. The engine is SQLite; other SQL dialects may differ.
Try this example in the SQL playground →Practise this idea
Show each customer id, name and number of orders as total, including zero-order customers. Order by customer id.
Check answer & explanation
SELECT c.id,c.name,COUNT(o.id) AS total FROM customers c LEFT JOIN orders o ON c.id=o.customer_id GROUP BY c.id,c.name ORDER BY c.id;LEFT JOIN keeps customers who have no orders. COUNT(o.id) excludes their unmatched NULL and returns zero rather than incorrectly counting one joined row.
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Check your understanding
A foreign key…
Check the quiz answer
Links related tables
A primary key identifies a row; a foreign key links to another table. Normalization reduces duplicated facts. Keep customer details in customers and reference customer_id from orders. Check performance and real requirements before redesigning.
References & corrections
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