Explain database normalisation and the common normal forms.

Normalisation organises columns into tables to reduce redundancy and update anomalies. The common normal forms:

• 1NF: atomic values, no repeating groups, each row unique.
• 2NF: 1NF plus no partial dependency on part of a composite key.
• 3NF: 2NF plus no transitive dependency, so non-key attributes depend only on the key.
• BCNF: every determinant is a candidate key, a stricter version of 3NF.

Unnormalised: order(id, customer_name, customer_city, product, qty)
3NF: customer(id, name, city)
     product(id, name, price)
     order(id, customer_id)
     order_item(order_id, product_id, qty)

Each fact is stored once, so updating a customer's city touches one row. Normalisation improves integrity and simplifies writes, and it can cost joins on read. Denormalisation deliberately reintroduces redundancy, for example storing a total or a copied name, to speed reads, and must be maintained carefully with triggers or application logic.