Storing customer email in every order row.

Store customer details once in CUSTOMERS and reference the customer from ORDERS by customer_id.

If the customer changes an email address, multiple order rows now need updates instead of one customer row. Relational design exists to remove that redundancy.

Ask whether the value belongs to the entity itself or to a specific transaction.

High

This creates duplication, update anomalies, and conflicting customer data.

Assuming every database problem should use a relational database.

Use relational databases for structured, related, integrity-sensitive data, and evaluate non-relational systems only when the workload truly demands them.

Relational databases are the default for many business systems, but not every workload needs joins, strict schemas, or ACID-heavy writes. Choosing well matters more than repeating a blanket rule.

Start with the data model and access pattern before choosing the database family.

Medium

A poor database choice can create unnecessary operational and modeling complexity.