Explain Normalisation and Write Queries | Dataford Interview Questions

Explain database normalisation and its types, then write a few basic SQL queries

Asked in the Round 4 stage, after the candidate named database systems as a favourite course. Explain the main normalization levels, then write a PostgreSQL query using the provided normalized order data.

Output

One row per completed order with order_id, customer_name, order_date, and total_order_value.
Include completed orders without line items, showing a total of zero.
Order by total_order_value descending, then order_id ascending.

Schema

customers

Column	Type	Description
customer_idPK	INT	Unique customer identifier
customer_name	VARCHAR(100)	Customer display name

orders

Column	Type	Description
order_idPK	INT	Unique order identifier
customer_id	INT	Customer who placed the order
order_date	DATE	Date the order was created
status	VARCHAR(20)	Order lifecycle status

order_items

Column	Type	Description
order_item_idPK	INT	Unique order-line identifier
order_id	INT	Order containing the line item
quantity	INT	Number of units ordered
unit_price	NUMERIC(10,2)	Price per unit at order time

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