Missing foreign keys - "I'll just store customer_id without FK constraint"

Always add FOREIGN KEY constraint

Foreign keys enforce referential integrity at database level.

Foreign keys are not optional. They enforce data integrity.

High

Allows orphaned records (orders with non-existent customers)

Wrong relationship type - Using 1:N when M:N needed

Create junction table for many-to-many

If both sides can have multiple relationships, you need M:N with junction table.

Students↔Courses, Products↔Categories need junction tables

High

Cannot store multiple relationships, violates 1NF

Using CASCADE everywhere

Choose CASCADE option based on business rules

CASCADE is dangerous. Use RESTRICT as default, CASCADE only when children are meaningless without parent.

RESTRICT is safest. CASCADE only for dependent data.

High

Accidental deletions cascade and delete large amounts of data