Keys and Joins in SQL | Dataford Interview Questions

What are primary and foreign keys? Explain the joins in SQL.

Asked in the Technical interview 1 stage. Write a PostgreSQL query that demonstrates these relationships by summarizing account activity for every customer.

Output

One row per customer with customer_id, customer_name, account_count, and total_transaction_amount.
Include customers without accounts or transactions, showing zero for both measures.
Treat NULL transaction amounts as contributing zero, and order by customer_id ascending.

Schema

customers

Column	Type	Description
customer_idPK	INT	Unique customer identifier
customer_name	VARCHAR(100)	Customer's full name

accounts

Column	Type	Description
account_idPK	INT	Unique account identifier
customer_id	INT	Customer who owns the account
account_type	VARCHAR(30)	Type of account

transactions

Column	Type	Description
transaction_idPK	INT	Unique transaction identifier
account_id	INT	Account associated with the transaction
amount	DECIMAL(12,2)	Transaction amount

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Problem