Storing comma-separated values: products VARCHAR(200)

Create ORDER_ITEMS table with product_id foreign key

Comma-separated values prevent atomic queries, violate 1NF, make updates error-prone.

Always use separate rows or junction tables for multiple values

High

Violates 1NF, cannot query individual products efficiently

Denormalizing without measuring performance first

Profile queries, identify bottlenecks, then denormalize specific tables

Denormalization trades integrity for speed. Only do it when measurements prove necessity.

Normalize first, denormalize only when profiling shows bottleneck

Medium

Premature optimization adds complexity without proven benefit

Missing foreign keys between related tables

Add FOREIGN KEY constraints: FOREIGN KEY (customer_id) REFERENCES customers(customer_id)

Foreign keys enforce relationships, prevent invalid data, enable CASCADE operations.

Always add foreign key constraints for relationships

High

Allows orphaned records, no referential integrity